Electron spin resonance spectroscopy has been used to detect the photoionization yield of meso-tetraphenylporphyrin (H2TPP) in SiMCM-48, AlMCM-48, and TiMCM-48 mesoporous molecular sieves. MCM-48 materials are shown to be effective heterogeneous hosts to accomplish long-lived photoinduced electron transfer of bulky porphyrin molecules at room temperature. TiMCM-48 and AlMCM-48 have higher photoyields of H2TPP(+) radicals than does SiMCM-48, indicating that the incorporation of Al and Ti into MCM-48 frameworks enhances the electron-accepting ability of the framework. H2TPP+ radicals are more stable in TiMCM-48 than in SiMCM-48 and AlMCM-48.