The photoionization of N-alkylphenothiazines (PCn where n = 1, 3, 6, 10, and 16) in mesoporous silicoaluminophosphate (UHM-3) and transition metal-substituted silicoaluminophosphate (MUHM-3 where M = Cu, Ni, Cr, and Mn) molecular sieves has been studied by electron spin resonance with ultraviolet irradiation at room temperature. Mesoporous UHM-3 and MUHM-3 molecular sieves were synthesized at room temperature. The effect of the ratio of Si/Al, the nature of the transition metal, and the concentration of the metal on the photoyield was examined. N-Alkylphenothiazine was incorporated into these mesoporous materials and the photoionization of PCn was studied.