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The photoionization of methylphenothiazine in microporous titanosilicates M-ETS-10 (M \ Na`] K`, H`, Li`, Na`, K`, Ni`, Cu2`and Co2`) and Na,K-ETS-4 molecular sieves with UV irradiation at room temperature is studied. Methylphenothiazine cation radicals are produced in M-ETS-10 molecular (PC`) sieves and are characterized by electron paramagnetic resonance (EPR) and di †use reÑectance (DR) UV-vis spectroscopy. Microporous titanosilicate Na,K-ETS-10 and its ion-exchanged molecular sieves with alkali and transition metal ions are shown to be efficient heterogeneous hosts to accomplish stable net photoinduced electron transfer from methylphenothiazine molecules at room temperature. The photoyield of cation PC 1 radicals was enhanced by incorporation of protons into ion-exchange sites of Na,K-ETS-10. The photoionization efficiency of methylphenothiazine cation radicals decreases in the order H-ETS-10 [ Na,K-ETS-10 [ K-ETS-10 [ Na-ETS-10 [ Ni-ETS-10 [ Li-ETS-10 [ Cu-ETS-10 [ Co-ETS-10. For Na,K-ETSno radical is detected either by EPR or by DR spectroscopy. As the alkyl chain length of the 4ÈPC 1 , PC 1 phenothiazine increases from methyl to hexyl the photoionization yield of H-ETS-10 decreases. Thermogravimetric analysis results support this trend. It is found that the photochemistry in M-ETS-10 and Na,K-ETS-4 molecular sieves is sensitive to the metal ion, pore size and internal void space.