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Adsorption isotherm data have been determined at 295 K for salicylic (SA), 4-aminobenzoic (ABZA) and 3chloro-4hydroxybenzoic (CHBZA) acids onto TiO, particles suspended in dilute aqueous solution. Good fit is achieved between these dark-adsorption data and linearised isotherm expressions based on competitive adsorption-desorption equilibria involving competition between solute and solvent. Such linear analyses yield values for K, the equilibrium constants, and also for (n",,,,,, , the limiting extents of solute adsorption, attainable in competition with the solvent. The latter values do not correspond to occupancy of each surface 0-Ti-0 site by a single adsorbed solute, but rather indicate that, on average, each adsorbed acid molecule coexists with ca. 40 adsorbed H , O molecules. Concentration dependences predicted for rates of photodegradation of SA, ABZA and CHBZA on the basis of the experimentally determined adsorption parameters, plus an assumption that a pseudo-first-order Langmuir-Hinshelwood-type rate expression applies to Ti0,-sensitised photodegradations, differ markedly from initial rates observed for the latter at 297.5 k 2.5 K. Consideration is given to various mechanistic sources of such discrepancy.