Methylphenothiazine and other N-alkylphenothiazines were introduced into silica gel pores by impregnation and sol-gel synthesis. The alkylphenothiazines were photooxidized at room temperature by 320-nm irradiation to form stable alkylphenothiazine cation radicals detected by electron spin resonance and diffuse reflectance spectroscopies. The silica gel framework is suggested to be the electron acceptor. The photoyield and stability of the methylphenothiazine cation radical depend on the silica gel pore size. In a small-pore silica gel the methylphenothiazine cation radical has a larger photoyield and is more stable with a longer lifetime than in a large-pore silica gel. Increasing the alkyl chain length up to hexadecyl does not affect the alkylphenothiazine cation photoyield or stability. However, addition of an anionic sulfate group to the alkyl chain of an alkylphenothiazine doubles its photoyield and increases the stability of the photoproduced radical. It is suggested that the mobility of the alkylphenothiazine cation radical in the silica gel pores controls its photoyield and stability. The alkylphenothiazine molecules in a large pore size silica gel are photooxidized more effectively at 77 K than at room temperature. Impregnation and sol-gel synthesis are equally good for introducing alkylphenothiazine molecules into silica gel for efficient photooxidation.