The dissociation of surface hydroxyl groups of crystalline SiO2, ZrO2, and ThO2, in aqueous suspensions, has been studied as a function of pHs at different ionic strengths of KNO3. The surface groups of quartz dissociate as weak acids, while those of ZrO2 and ThO2 dissociate amphoterically. A reversible double layer is formed at the oxide–solution interface, and H+ and OH− function as the potential-determining ions. Quantitative data have been obtained on (a) surface charge densities, (b) zero point of charge, (c) differential capacities of the double layer on quartz and ZrO2, and (d) the effect of surface charge density and ionic strength on the interfacial tension. The differential capacities indicate specific adsorption of NO3−on ZrO2 and ThO2 while NO3− has zero affinity for quartz surfaces. At low negative charge densities, solvated K+ are adsorbed on quartz and ZrO2 through coulombic interactions while at pHs > 10 specific adsorption of K+ predominates. ThO2 appears to exhibit a greater tendency for the specific adsorption of K+.