The interaction of catechol and water with titanium oxide nanoparticles was investigated using ab initio molecular orbital theory and density functional theory. Hydrogen-terminated TiO2 clusters were used to model the surface of anatase nanoparticles. The calculations indicate that catechol reacts with a Ti=O defect site on the surface to form a bidentate structure that is favored over dissociative or molecular adsorption on the (101) anatase surface. The dissociative adsorption of catechol at the defect site leads to a much larger red shift in the TiO2 excitation energy than molecular adsorption on the (101) anatase surface on the basis of ZINDO/S calculations. This is consistent with recent experimental results on small (<2 mn) titania nanoparticles. The calculations on water adsorption indicate that it can also add to the Ti=O double bond site. However, molecular adsorption of water on the (101) anatase surface is more favorable.