The forces between a silica probe and silica and gold substrates were measured with an atomic force microscope in the presence of a series of alkali-halide electrolyte solutions. The interaction between two silica surfaces was repulsive and could be accurately predicted by Derjaguin-Landau-Verwey-Overbeek theory. The silica surface was negatively charged at a pH of 5.5, and the effective surface potential increased in magnitude with decreasing electrolyte concentration. In contrast, the force between the silica probe and a gold substrate was attractive at open circuit. This interaction was a strong function of the potentia applied to the gold and the nature of the electrolyte species. In general, the silica-gold interaction changed from attractive, when the gold was held at positive potentials, to repulsive at negative potentials. A series of force measurements as a function of the potential of the gold electrode indicated that the repulsive force increased when moving toward more negative potentials, corresponding to the removal of adsorbed anions. The potential at which the silica-gold interaction passed through a minimum, referred to as the potential of zero force (pzf), corresponded closely to the potential of zero charge (pzc) in these systems. The pzf values were compared to those determined from measurements of the electrode capacitance in 10(-3) M solutions of NaF, KCl, KBr, and KI. The force data were also compared to theoretical predictions of the forces between dissimilarly charged surfaces obtained by solving the complete nonlinear Poisson-Boltzmann equation.