Chromium ion-exchanged zeolite X microporous materials were examined for the photoionization of N,N,N'N'-tetramethylbenzidine (TMB) with 320-nm irradiation at room temperature. Photoionization of chromium-containing zeolite X results in the formation of N,N,N'N'-tetramethylbenzidine radical cations (TMB+.), which are characterized by electron spin resonance (ESR) and diffuse reflectance spectroscopy (DRS). ESR studies indicate that Cr(V) acts as an electron acceptor. The photoyield and stability of the photoproduced TMB+. cation radicals is higher in Cr(V)-X zeolite than in other hosts such as silica gels, zirconium phosphate, mordenite, and silicoaluminophosphates. The study delineates the importance of the size/shape features of the host system on the stability of the photoproduced TMB+. cation radicals.