An analysis of surface potential nonlinearity (psi(0)) at metal oxide/electrolyte interfaces is presented. By using grand canonical Monte Carlo simulations of a simple lattice model of an interface, we show that a correlation exists between ionic strength, as well as surface site densities, and the non-Nernstian response of a metal-oxide electrode. We propose two approaches to deal with the psi(0)-nonlinearity: one based on perturbative expansion of the Gibbs free energy and another based on the assumption Of the pH dependence of surface potential slope. The theoretical analysis based on our new potential form gives excellent performance in extreme pH regions, where classical formulae for psi(0) are unjustified. The new formula is general and independent of any underlying assumptions. For this reason, it can be directly applied to experimental Surface potential measurements, including those for individual Surfaces of single crystals, as we present for data reported by Kallay and Preocanin [6]. (C) 2009 Elsevier Inc. All rights reserved.