An optical method is presented that allows simultaneous determination of the diffusion constant and electrophoretic mobility of individual charged particles with radius down to 0.2 pm. By this method the size dependency of the effective charges and zeta potentials of individual particles can be investigated, as well as interparticle interactions and Brownian motion in confined geometries. The diffusion constant and mobility are determined from the power spectrum of the particle speed in a sinusoidal electrical field. The accuracy of the method was tested on PMMA spheres of known size in water. Experiments have been carried out on charged pigment particles with low concentration in a nonaqueous medium containing a charging agent. The mobility is found to be independent of the particle size. (c) 2006 Elsevier Inc. All rights reserved.