The D.C. Corporation Counsel's Office has begun an investigation to determine if the chairman of the city board that rules on police and firemen's pensions held an invalid appointment for four months and whether cases acted upon during that time must be reconsidered.

James E. Nichols, an employe of the D.C. Office of Personnel, was appointed chairman of the Police and Fireman's Retirement and Relief Board in January by then-Personnel Director George R. Harold. Nichols replaced Percy Battle, who retired.

The formal order by Mayor Marion Barry making the appointment effective was not made until May 30, however. Aides to the mayor said yesterday that, as a result, there is some question as to whether any actions taken by Nichols as a board member prior to May 30 can be considered proper.

Acting Corporation Counsel Judith W. Rogers said yesterday that she did not know how many cases had been considered by the Board during that time, but she said she expects to report to the mayor soon on the validity of Nichols' appointment.

Mayor Barry said yesterday that he did not believe the appointment by Harrod in January was improper. "It doesn't appear to be illegal," Barry said. "I don't see any big problem."

It has not been uncommon in some instances for persons to continue serving as members of many of the move than 100 boards and commissions for years beyond their legally recognized terms until replacements have been chosen.

Nichols, however, was not on the board prior to his appointment in January, Barry aides said, and therefore his taking part in official board actions prior to the formal appointment in May may be open to question.

Assistant Corporation Counsel George Porter, who sits on the seven-member board, completed a "working paper" this week on the status of Nichols' appointment. The paper concludes that Nichols was a "de facto" member of the board from January to May and his actions were valid, Rogers said.

But, she added, that document was not an official corporation counsel's opinion and should not be considered a binding legal statement.

The board has been the focus of periodic attention over the last 18 months because of the high proportion of city police and firemen who have retired under the District's generous and controversial disability retirement system.