Adsorption of 2-nitrophenol and 4-nitrophenol by titanium dioxide was measured and their photocatalytic oxidation was followed by both HPLC analysis and measurement of the total organic carbon content of the solution. The oxidation follows pseudo-first order kinetics with rate constants that decrease as the nitrophenol concentration increases. However the adsorption constants inferred from the reaction kinetics are inconsistent with those measured in the adsorption experiments, even though the relative amounts of reaction intermediates remain low. We conclude that the decrease in rate constant cannot be explained by a Langmuir-Hinshelwood model. Instead we suggest that the strong, but differing, UV absorption by nitrophenols lowers the effective UV flux at the catalyst surface and that the greater absorption of more concentrated solutions is primarily responsible for the apparent decrease in rate constant. A simple treatment of this absorption adequately describes the observed changes in rate. The quantitative effect of UV absorption depends on the lamp and on the reactor optics. It his suggested that variations are responsible both for differences in the reported relative oxidation rates of the two isomers, and for the different conclusions reported with respect to the effect of pH on oxidation rate.