A high precision cylindrical resonator has been employed to study the effects of impurity gases on the molecular absorption in oxygen. The resonator is capable of handling temperatures up to 500°C and pressures up to 100 atmospheres covering an effectivef/prange from 10 to 250 000 Hz/atm. The effects of4HeandCO2gases on the molecular absorption in oxygen within these experimental regions will be presented and compared with previous measurement using other techniques, such as the direct absorption method [Bass, J. Chem. Physics,58, 4783 (1973)] and shock interferometry [Millikan and White, J. Chem. Phys.39, 3209 (1963)]. [Work supported by NASA.]