In this work we analyze the electrophoretic mobility of monodisperse colloidal spheres of zinc sulfide as a function of pH for different concentrations of solutions of Zn(NO3)(2) and (NH4)(2)S. The effect of surface oxidation on the electrokinetic properties of ZnS is also analyzed by comparing the pH dependence of the mobility of the synthetic particles with that found for commercial ZnS (both pure and oxidized by dispersion in 3 and 15% H2O2 solutions) and ZnO. Our results show that, although both Zn2+ and S2- (actually HS-) considerably affect the electrokinetic behavior of ZnS, a more significant effect is found for Zn2+ on the mobility of the colloidal particles. Oxidation of the commercial ZnS colloids considerably changes the mobility-pH trend of variation, increasing the pH corresponding to the i.e.p. and modifying the shape of the mobility-pH curve as compared to unoxidized samples. On the basis of electrokinetic and X-ray photoelectron spectroscopy measurements, it is suggested that the oxidation leads to the formation of surface elemental sulfur in the case of our synthetic spheres, and of zinc oxide and/or sulfur oxide species in the case of commercial powder.