The electrophoretic mobilities of copper phthalocyanine particles, dispersed in isoparaffin solutions containing zirconium octanoate, have been determined using phase-analysis light scattering. All the samples studied contained trace concentrations of water. The mobility values were converted to zeta potentials using the Huckel equation. All the systems studied exhibited a pronounced maximum in zeta potential as the zirconium octanoate concentration increased. The maximum occurred at a bulk zirconium octanoate concentration equivalent to that required for complete coverage of the particles. The zeta potential data were converted to surface charge density values through the use of the Poisson-Boltzmann equation. The latter were in the range 0.4 to 2.5 mu C m(-2). A simple two equation site binding theory, which considered the dissociation of zirconium octanoate and the subsequent adsorption of ions at a generic surface site, was successfully applied to the surface charge data. It is proposed that the maximum in the zeta potential and surface charge as a function of zirconium octanoate concentration was observed due to the preferential location of ZrO2+ ions at the particle surface, followed by charge neutralization with octanoate anions. It is suggested that water facilitates the dissociation process of the zirconium octanoate, although it does not directly contribute to the surface charge itself. Two plausible qualitative mechanisms are described. The first involves the presence of water at the particle-solution interface, whilst the second considers the formation of micelles in the bulk isoparaffin phase.