Accurate measurements of the InP refractive index as a function of free‐carrier doping are reported at 1.3 and 1.5 &mgr;m, the two strategic wavelengths for optical communications. A total of 21 samples with differentN‐ andP‐doping levels have been measured using a novel and simplified grating‐coupling technique. In contrast to the conventional method, this only involves the use of a directly etched diffraction grating on the sample surface, thereby avoiding the necessity of a specific guiding layer. The measured index, in agreement with earlier predictions, decreases by more than 0.05 when theNdoping is increased from below 1015to about 1019electrons per cubic centimeter. This effect, however, is much less pronounced withPdoping than withNdoping. ©1996 American Institute of Physics.