An open cell photoacoustic (PA) configuration was employed to evaluate the thermal diffusivity of intrinsic InP as well as InP doped with tin and iron. Thermal diffusivity data were evaluated from variation of phase of PA signal as a function of modulation frequency. In doped samples, we observe a reduced value for thermal diffusivity in comparison with intrinsic InP. We also obsd. that, while the phase of the PA signal varies linearly with the square root of chopping frequency for doped samples, the intrinsic material does not exhibit such behavior in the exptl. frequency range. These results were interpreted in terms of the heat generation and phonon assisted heat diffusion mechanisms in semiconductors.