Basic groceries for the family pantry can cost twice as much at Washington area convenience stores as they do at standard neighborhood supermarkets, according to a survey yesterday of the chain stores.

The price check focused on what shoppers pay for a baker's dozen selection of food and other common grocery items at such small convenience stores as 7-Eleven and High's, compared with sprawling suburban supermarkets such as Safeway and Giant.

The 7-Eleven almost consistently charged higher prices than the supermarkets. Prices at High's, although lower than 7-Eleven, were higher than the supermarkets on most items.

Two exceptions were milk and margarine, which cost less at High's than at 7-Eleven, Safeway or Giant.

Prices at the small convenience stores are higher because of their longer hours, smaller inventories and slower turnover of merchandise, store officials said yesterday.

Many 7-Eleven stores, for example, remain open 24 hours a day. Until recently, many supermarkets in the suburbs stayed open 24 hours a day. They now close at midnight in many locations, however.

The most extreme instance of price differences was in the bread section of the stores. A loaf of bread at 7-Eleven cost 67 cents, compared with 34 cents at Safeway and 36 cents at Giant.

High's was lower than 7-Eleven with its 59-cent bread price -- still substantially higher than the supermarket price.

Other striking examples of price variations included:

Eggs, 16 percent higher at the 7-Eleven and 12 percent higher at High's than at supermarkets.

Instant coffee 24 percent higher at 7-Eleven and 14 percent higher at High's.

Ice cream, 40 percent higher at 7-Eleven and 8 percent more expensive at High's than at the supermarkets.

"We try to be competitive on some items, such as milk," said Jerry Baker, manager of the 7-Eleven store at 3901 Lee Hwy., Arlington. Hence, the $2.05 for a gallon of homogenized milk at his store is the same as the price charged at the nearby Safeway and Giant.

"But our convenience items are higher," Baker said.

In charging higher prices, the more than 300 7-Eleven stores operating in the Washington area are carrying on a marketing philosophy that dates back to 1927 when the parent company opened its first outlet.

"Our purpose was to give the customer what he wants when he wants it," said Jean Miller, a representative of the Southland Corp., which owns and operates 7-Eleven."This was predicated on extended hours in neighborhood locations with stores carrying items that people want to buy."

Muller said that even today, despite sharply higher grocery prices, "most customers don't mind paying a few cents more if they can get it faster than standing in line at a supermarket or when everyone else has closed for the day."

"I know I can probably get three loaves of bread at Giant for each one I buy here," said Norman Haley, a Fairfax roofer who was shopping yesterday at a 7-Eleven in Arlington.

"But I come anyway -- because it's quick.'"

For this price comparison, the four stores checked were located in Arlington. But the same prices prevail throughout the Washington area, as a general rule, since the chains have a one-price policy on most basic goods but some local variations do exist.

Safeway operates about 130 stores in the metropolitan area; Giant, about 100 and High's about 300.

Dave Wenley, manager of a High's store at 3123 Lee Hwy. in Arlington, said his prices rarely are more than 20 cents higher than supermarket prices even with the added overhead expenses due to longer hours and slower turnover of merchandise.

Wenley and Baker of the Lee Highway 7-Eleven said their customers are willing to pay that extra charge for the convenience of using the stores.

"They can come in, buy what they want and be out in two minutes," Baker said.

Part of this ease is due to the fact that parking lots at the convenience stores are small so that customers can park nearly at the front door. Once inside, the stores are compact rather than spread out and rambling over thousands of square feet.