The efficiency of photo-induced charge separation in smooth meso-tetra(alkylphenyl)porphyrin/TiO2 bilayers has been investigated using the Time-Resolved Microwave Conductivity (TRMC) technique. Particular attention has been paid to the effect of the structure of the substituents on the charge separation efficiency. The chemical structure of the substituents is found to affect the charge separation efficiency significantly. Ethylsubstituted meso-tetraphenylporphyrin/TiO2 bilayers exhibit a charge separation efficiency of 6.2% at the absorption maximum, which is considerably higher than the efficiencies near 1% or even lower found for the other porphyrin derivatives. The low charge separation efficiencies of these porphyrin/TiO2 bilayers are discussed in terms of a short exciton diffusion length and/or a low interfacial electron injection yield (phi (inj)). The short exciton diffusion lengths are related to a short lifetime of the singlet state and a low energy transfer rate between porphyrin molecules due to a low fluorescence rate constant. The results suggest that the value of phi(inj) is affected by the presence of alkyl substituents on the phenyl groups. (c) 2005 Elsevier B.V. All rights reserved.