Tetra(4-carboxyphenyl)porphyrin (TCPP) adsorbs strongly onto nanoparticulate TiO2 and serves as an efficient photosensitizer for solar-energy conversion by TCPP-sensitized TiO2 electrodes. Nanoparticulate TiO2 electrodes were prepared from Degussa P25 TiO2 powder in the standard manner for a Gratzel cell. Adsorption studies of TCPP onto these sintered TiO2 electrodes gave a saturation surface coverage of 47 mu mol/g. Adsorption studies of TCPP onto colloidal dispersions of Degussa P25 in ethanol Save a saturation surface coverage of 77 mu mol/g. The difference between the saturation coverages is attributed to the reduction of the available surface area in the TiO2 films after sintering, from 55 m(2)/g as a free colloid to about 34 m(2)/g as a sintered electrode. The nature of the binding of TCPP onto the TiO2 electrodes was investigated using X-ray photoelectron spectroscopy (XPS) and Resonance Raman Spectroscopy (RRS). In the XPS spectra of TiO2 with adsorbed TCPP, the O (Is) and Ti (2p(3/2)) peaks of TiO2 were shifted to a higher binding energy value, by about 0.3 eV, and the O (Is) and N (Is) peaks of TCPP were shifted to a higher binding energy, by about 0.7 eV. Upon adsorption of TCPP, one of the Ti (2p3/2) peaks of TiO2 disappeared, suggesting complexation and removal of surface states. The RRS results indicated that for cases in which TCPP was adsorbed onto TiO2 films from ethanolic solutions of about 1 CIM concentration, the porphyrin spectrum showed distinctive interactions with the surface, but for cases in which it was adsorbed from higher concentrations, the RRS spectra were similar to spectra of TCPP powder, indicating the dominance of porphyrin-porphyrin interactions. We conclude that lateral interactions between adsorbed TCPP are significant upon adsorption from all but the lowest (micromolar) initial concentrations. Photovoltaic cells with TCPP-sensitized TiO2 electrodes gave good solar-energy conversion efficiencies. At light simulating one sun (AM 1.5), a cell sensitized by TCPP gives a short-circuit photocurrent of about 6 mA/cm(2) and an open-circuit photopotential of 485 mV. The incident photon-to-current conversion efficiency was 55% at the Soret peak and 25-45% at the Q-band peaks; the cells have a fill factor of 60-70% and an overall energy conversion efficiency of about 3%.