Professional serving here with international groups, whose tax-paid salaries already average $15,000 to $28,000 more than counterpart U.S government executives have just been given a 9.5 percent raise.

The new pay raise is a regular "catchup" with U.S industry. It is nearly double the 5.5 percent raise President Carter has set this October for U.S federal and military personnel.

That new boost will further widen the existing pay gap between rates for international civil servants and U.S bureaucrats who often perform similar jobs at the same level of responsibility. International employes, thanks to special tax breaks, can earn up to $50,000 more per year than their top-level U.S counterparts.

Locally about 8,000 people here - many of them Americans - work for health, welfare or financial associations of the U.N., Organization of American States or other multinational or international bodies. The operations generally are financed by member nations with the U.S often paying the largest proportion of operating costs.

By international agreement, most employes of the organizations are exempt from U.S taxes while working here. However, since Americans must pay taxes their international employers reimburse them for all federal and state taxes, and Social Security payments.

Officials at the State and Treasury Departments who keep tabs on international organizations are reluctant, for diplomatic or political reasons, to discuss pay systems of the groups. Congressional insiders, and auditors for the General Accounting Office can and will talk about what they have dicovered in overnight studies.

Some examples:

Salaries paid Americans working for the International Monetary Fund, Inter-American Development Bank and similar groups are "grossed up" to compensate those workers for local and federal taxes, and Social Security contributions.

Net salaries are "grossed up" individually, depending on the marital status, number of dependents and residence (for tax purposes) of the American emplye of the international orgaization.

For example, executives might be in the "net" salary range of $31,980 to $41,730. Those are jobs that are roughly equivalent to civil service pay rates at Grade 15, which range from $38,160 to $47,500

After the international organization has the salary "grossed up," the actual payout to employes whose net income is listed as $31,980 to $41,730 would in fact be between $53,530 and $75,660. The "difference" is to reimburse the employe for taxes and Social Security.

Since "grossed up" pay of international organization employes is figured on the basis of the individual's federal and state tax liabilities based on standard deductions, many workers who itemize actually are overcompensated for taxes, since they often pay less by itemizing.

Under the "grossed up" system, some professional level jobs at the IMF actually pay from $38,410 to $108,030. At the Inter-American Development Bank, the rates range from $30,270 fir junior level professionals to $91,000 for the executive vice president. Comparable pay for top U.S. Treasury Secretary gets $66,000.

A Senate aide familiar with the complicated international pay scales says many UN. salaries in New York. Geneva and other cities are even higher than in Washington. He admitted that it is difficult to compare international employes with American government executives, since the international aides have little or no tenure, no job appeal rights and often must have second and third language qualifications U.S. government workers do not need.

At any rate, insiders say an over-haul of the complicated tax-reimbursing system appears to be in the wind. In the meantime, international bureaucrats appear to be world's top-salaried government executives by any measure.