A Malvern Zetasizer IIc laser Doppler electrophoresis apparatus has been modified to enable the effect of illumination on electrophoretic mobilities of aqueous dispersions of semiconductor particles to be studied. The electrophoretic mobilities of TiO2 particles were determined in the dark and whilst exposed to monochromatic light, the wavelength of which was varied between experiments. The results showed that a positive charge accumulated on the particles in the absence of easily reducible or oxidizable species in solution. This effect was observed only when the particles were illuminated with photons of ultra-band gap energy, demonstrating unambiguously that it was related to the generation of electron-hole pairs and their subsequent reactions with Ti-O surface groups. Similar shifts were observed for suspensions of SnO2, Ti4O7 and Fe(CN)64- photosensitized TiO2 particles. No such shift was observed for CdS particles in the absence of any excess lattice ions, probably owing to decomposition of the particles to generate a surface layer of photo-inactive sulphur.