Looking back, Robert Korman can count off 18 years on the job at Mobil Corp.

But he has another number now, something between a 1 and a 5, that will bear heavily on his future when the $81 billion combination of Mobil and Exxon Corp. becomes final.

In preparation for the largest oil merger in history, Exxon and Mobil executives and supervisors have rated the two companies' combined 120,000 employees worldwide, placing them into five groups based on their performance, employees say.

For the 2,000-plus Mobil employees at its Fairfax headquarters, the ratings will help determine who gets to stay, who is asked to move to the new company's headquarters in Irving, Tex., or other locations--and who is asked to leave.

And like the others, Korman is waiting to find out what his number is and where he stands. The 20 staffers in Korman's department in Fairfax, which swaps oil between Mobil and other producers, were rated a second way, from top to bottom.

"I wasn't told whether I was 1 or 10 or 20," Korman said. "I'm like everyone else: I'm up in the air."

Korman says he isn't particularly worried about the outcome, but other colleagues are.

After completion of the merger--regulatory approval could come as soon as the next few days--the gleaming Mobil office complex along Gallows Road will be transformed from a corporate headquarters into the refining and marketing command center for the new Exxon Mobil Corp.--a big operation, but not the home base anymore.

Some Mobil workers fear that hundreds of employees with "corporate headquarters" tasks may face a move to Texas, or a buyout.

"There are a lot of accountants, lawyers, planners, public relations and government relations people" in Mobil's Washington area offices, said a former Mobil employee who still works at headquarters on a contract basis. "Those people are the most prone to have overlap with Exxon."

The waiting has been hard, some employees say.

Since the merger announcement Dec. 1, transition teams made up of officials of both firms have been at work on plans to splice the companies together.

"There's a lot of anxiety," said one longtime employee who works in Mobil's global fuels division. "Everyone in the company expected [the merger to be approved] at the end of the third quarter" two weeks ago. "Everyone asks, 'Have you heard anything?' It's at the water cooler. It's at lunchtime."

A Mobil engineer who has worked for the company for three decades adds: "We've been living with it since last Thanksgiving--the apprehension, the unknown. We've been living with it so long that we just want closure. On the whole, most people are quietly anxious." These workers asked that their names not be published.

Exxon and Mobil have said little publicly about their post-merger plans. Spokesmen for Mobil and Exxon declined to discuss these plans for this article.

But Mobil employees say they have been reassured by a buyout offer they describe as generous. According to employees, Mobil says employees can refuse a transfer of more than 50 miles, such as to Texas, and then accept a severance buyout equal to four weeks' pay for every year worked at Mobil up to 26 years' employment.

Indeed some employees say their greatest fear is that they won't be offered the buyout, and will wind up in a job at Fairfax for a few years, only to face more downsizing later on.

If a Mobil worker refuses a transfer out of the Washington area and then turns down a subsequent job offer in Fairfax, the worker may not be eligible for the buyout. (Exceptions may be made when the new offer is at a much lower level than an employee's previous job.)

The buyout could mean that some employees with 26 years or more at Mobil could walk away with two years' pay, a provision that longtime workers in their fifties and sixties say they find especially appealing.

"I'm an older employee," said Jeff Chenen, in Mobil's real estate group. "Mobil has been very straightforward. They've made some fair arrangements with us. I think it's positive for most employees."

Korman said he hopes to stay at the new firm, although under the buyout he would get 72 weeks' severance. If he should lose his job, Korman said he is not worried about the future.

"I have confidence in my skills," said Korman, who has a graduate degree in finance from Villanova. "Mobil people are looked upon very favorably. A Mobil education is valued in the workplace.

"I'll end up okay one way or the other."

That was a sentiment echoed by the Mobil engineer, who said he had mixed feelings about his yet-to-be-determined job fate.

"I'd be happy with the new organization," he said. "But I'd be happy to take the [severance] package" with two years' pay. "I'm probably going to land on my feet either way."

Decision day has been a long time coming for Exxon and Mobil employees. Although European regulators have approved the merger, the Federal Trade Commission's antitrust division has pushed the oil companies for months to agree to sell a significant portion of their gasoline retail business and other operations, to prevent the combined companies from having too much market power. Sources close to the talks say an agreement with the FTC calling for the combined companies to sell off more than 1,500 service stations is near.

Beyond the reorganization imposed by regulators, the two companies themselves must work out a hugely complex intermingling and pruning of their organizations, said oil industry analyst Fadel Gheit at Fahnestock & Co.

Heading the problems is what happens when a Mobil and an Exxon executive or employee are in competition for the same spot. At the very top, the decision has already been made: Exxon's chief executive, Lee R. Raymond, will head the new firm, while Mobil's boss, Lucio A. Noto, will become vice chairman. Analysts predict that Mobil's president, Eugene A. Renna, will take over the refining and marketing operations in Fairfax.

How jobs will be split among Mobil and Exxon employees remains to be seen, and some Mobil employees say they fear the edge will go to people from Exxon, which is twice Mobil's size. "Mostly it will be the Exxon people who will prevail," the Mobil contract employee predicted.

One ex-employee who has kept in close contact with former colleagues said, "Exxon has taken over, and it's clear Exxon is calling all the shots. No department head does anything without calling a counterpart at Exxon. People are very nervous. You know, to the victor goes the spoils."

But Gheit said his contacts at Mobil and Exxon assure him that selections will be based heavily on candidates' track records, not Exxon's bigger size, because the companies need the best qualified people.

If, as he believes, Mobil is stronger in refining and marketing, their people should prevail in those assignments. Exxon is the leader in finance, chemicals, exploration and production and will get more of the top positions there, he predicted. In Europe, Exxon has the edge, but Asia is more Mobil's bailiwick, he said, and that should guide appointments in those areas.

Given those differences, it's possible that some Mobil employees have a much clearer picture of the future than others, and perhaps that's why assessments of morale at the Fairfax headquarters vary so.

Waiting has had a negative impact, Gheit said. "If I do not know whether I will be employed by the company, I'd be crazy not to look for another job. Even if I get assurances I'll be employed, I don't know who my boss is. I don't know whether I'll like the boss. That's why the anxiety level is incredible."

The former Mobil employee who now does contract work described the headquarters as "a depressing place."

"You've got a lot of people who are terrified," the contract employee said. "Everybody's waiting for the shoe to drop."

But another Mobil employee who recently left the company said that anxiety has been at a relatively low level. Noto and Renna have made themselves available at employee forums worldwide, taking questions, he said: "To Mobil's credit, they have worked very, very hard doing everything they could to try to keep people informed."

And as for anxieties, the economy has done a lot to keep those low, he said. "Their payout in dollars will be very good, if, in fact, they're one of those people who loses a job. And a lot of those people live in Fairfax," he said.

"And what's the unemployment rate in Northern Virginia? Two percent?"

Staff writer Martha M. Hamilton contributed to this report.

CAPTION: Once the Exxon-Mobil merger is complete, Mobil's corporate headquarters complex on Gallows Road will become the new company's refining and marketing command center.