We report studies directed toward a quantitative understanding of the relationship between the flatband potential (V-fb) of a metal oxide semiconductor electrode (titanium dioxide) and the composition of a contiguous electrolyte solution (water, methanol, ethanol, and acetonitrile each containing 0.2 mol dm(-3) tetrabutylammonium perchlorate). A relationship between V-fb and the proton-generating ability of the electrolyte solvent, as measured by the autoprotolysis constant of the pure solvent, is demonstrated. Additional studies have examined the dependence of V-fb on the composition of a binary electrolyte solvent mixture (acetonitrile/ethylene carbonate and acetonitrile/water). We conclude that the composition of the acetonitrile/ethylene carbonate mixture at the semiconductor-electrolyte solution interface (SLI) is similar to that of the bulk solution. However, for an acetonitrile/water mixture there is preferential adsorption of water at the SLI. Finally, for aprotic electrolyte solutions, the dependence of V-fb on the electrolytic solute was studied. Cations such as Li+ and Na+ are potential determining.