Metal silicoaluminophosphate, M-SAPO-n, (M = Cr, Fe or Mn, It = 5, 8, 11) microporous materials with incorporated methylphenothiazine are photoionized with 320 nm light at room temperature. Methylphenothiazine cation radicals are produced and characterized by electron spin resonance. Transition metal ion containing microporous M-SAPO are found to be efficient hosts for the formation and stabilization of methylphenothiazine cation radicals (PC1+). The photoyield is either negligible or not observed in the case of H-SAPO-n materials, indicating that the transition metal ions serve as suitable electron accepters and assist in the stabilization of the photoproduced PC1+ radicals. Chromium containing SAPO materials exhibited the highest photoyield among the Cr, Fe, and Mn transition metal ions studied. The photoionization efficiency was found to depend on the nature of the metal ion, the concentration of the metal ion, and also on the pore size of the microporous SAPO material used.