The surface photovoltage spectra of InP and Zn3P2 were measured using a Kelvin probe to determine the contact potential difference between the sample and the probe as a function of the wavelength of illuminating light. The features in the resulting spectra were found to be sensitive to ion bombardment. The photovoltage spectra obtained from the InP differed from previously reported SPV spectra in that it showed clear evidence of surface states (or interfacial states) at 0.86 eV and 0.68 eV above VBM. It was found that the features in the spectrum of Zn3P2 were reduced by ion bombardment, but not removed completely, whereas the features in the InP spectra were completely removed. Exposure of the ion bombarded surface to air restored the features on Zn3P2 but only produced a small change in the spectrum of the InP. The loss of features in the InP spectra can be attributed to damage in the substrate caused by the ion bombardment even though the oxide layer was not removed before the damage occurred. Zn3P2 was not as sensitive to ion damage as InP.