Hall data on vapor‐phase‐epitaxial InP doped with Si, S, Sn, and Se and liquid‐encapsulated‐Czochralski InP doped with Ge are presented for doping levels between 1015and 1019cm−3. The results show nearly identical electrical behavior for alln‐type dopants but a consistent discrepancy with theoretical mobility calculations, particularly in the doping range 1017–1019cm−3. A chemical analysis of the sulphur content in layers doped with sulphur shows that all the dopant is electrically active. The reason for the discrepancy with the calculation is discussed in terms of the description of ionized impurity scattering in the presence of a high density of ionized centers. We also emphasize the similarity with GaAs and suggest that our conclusions apply equally to both materials.