Results from quantum chemical INDO/S-CI calculations of the absorption properties of anatase TiO2 nanoparticles sensitized by catechol and benzoic acid are reported. Experimental observations that catechol causes a strong shift in the TiO2 absorption threshold, while benzoic acid does not, are explained from the calculations which indicate that only catechol introduces electronic levels in the TiO2 band gap. The 420 nm excitation in the TiO2 + catechol system responsible for the shift is seen to be a charge-transfer excitation from the catechol pi HOMO orbital to Ti(3d) levels at the bottom of the conduction band. A direct photoinjection scheme which does not involve excited states of the adsorbate is proposed to account for the electron injection process.