Chloromethanes were added to silica gel of 14 nm pore size with methylphenothiazine (PC1) trapped in them. The methylphenothiazine was photoionized by 320-nm irradiation by a one-photon process to form stable methylphenothiazine cation radical (PC1+) which was detected by electron spin resonance. The addition of CCl4 or CHCl3 increases the net PC1+ photoyield when the samples are irradiated at room temperature while CH2Cl2 and CH3Cl have no effect. Thus, CHCl3 and CCl4 are suggested to act as better electron accepters in competition with the silica gel framework while CH2Cl2 and CH3Cl do not seem to be able to compete with the silica gel framework. A chloromethane with more Cl atoms has a greater electron affinity which supports this interpretation. No effects of these chloromethanes on the net photoionization yield of PC1 in silica gels at 77 K were observed, probably due to the immobilization of the chloromethanes at 77 K.