It is shown that photoreflectance (PR), a contactless form of modulation spectroscopy, can be used to measure the resistivity of semi‐insulating InP:Fe substrates. PR measurements of the substrates, obtained from various vendors and laboratories, were performed at 82 K and had line shapes dominated by excitonic transitions. The phase angle of the line shape was found to be a measure of the bulk resistivity of the substrate. The behavior of the line shape phase angle is explained by the exciton ionization model, where the electric field near the surface of the sample is sufficient to ionize excitons. The extent of this ionizing electric field corresponds to an optical path length which enters additively into the exciton line shape phase. Samples with higher resistivities have ionizing electric fields of larger extent, increasing the phase angle of the exciton line shape.