The photoreduction of methyl viologen (MV2+) was examined in zeolite X. A series of alkali metal ion-exchanged zeolite X materials with ion-exchanged methyl viologen was photoionized with 320 nin light at room temperature in the absence of any reducing counteranion. Photoreduction of methyl viologen containing alkali metal ion-exchanged zeolite X results in the formation of methyl viologen cation radicals (MV.+). The radicals were identified by electron spin resonance (ESR). Upon irradiation at room temperature the samples turn light blue in color and a single line ESR spectrum characteristic of the methyl viologen radical cation is observed. The photoyield depends on the nature of the alkali metal ion-exchanged into the zeolitc framework. The photoyield increases in the order Li-X/MV2+ < Na-X/MV2+ < K-X/MV2+ < Rb-X/MV2+ < Cs-X/MV2+. The donor strength of the zeolite framework increases in the order Li-X < Na-X < K-X < Rb-X < Cs-X. Thus, the electron donor is suggested to be the anionic aluminosilicate framework of the host zeolite. This is supported by a linear correlation of the photoyields of alkali metal ions exchanged into zeolite X with Sanderson's partial charges on the framework oxygens.