The sorption and desorption of Th onto TiO2 (10 gn) was studied as a function of pH (1-11), ionic strength (0.005-0.1 M NaCl, NaClO4), thorium concentration (from 1 x 10(-9) to 3 x 10(-2) M), and carbonate concentration (up to 10-2 M) using Th-234 tracer. The results indicate the reversible formation of an inner sphere complex with a strong pH dependence. No effect of the carbonate on sorption could be measured in the pH range investigated (5-10.5), The data were fitted with five different conceptual models (constant capacitance, diffuse layer, Stern, triple layer, and a nonelectrostatic model) using FITEQL. Different hydroxy complex surface reactions were tried, one at a time, varying the site density (1-12 sites/nm(2)). The reaction that provided the best fit depended strongly on the surface site density and the model used. To be able to fit a reaction to the pH-dependent data with the compact layer models, the effect of the electrostatic term had to be decreased by decreasing the site density below 5 sites/nm(2). None of the electrostatic models could be used to fit all of the isotherm data. The nonelectrostatic model could be used to reasonably fit both the pH-dependent and isotherm data.