Adsorption or intercalation of trans-1-methyl-4-(2-phenylvinyl)pyridinium ion (stilbazolium ion, SB (1)) to a polyelectrolyte, i.e., poly(potassium vinylsulfate) (PVSK), and to a hectorite clay is studied and its effect on the photochemistry of SE examined. trans-cis Photoisomerization of SB molecularly adsorbed to PVSK proceeds efficiently as with free SB in water, while that of SE molecularly intercalated in the clay is markedly suppressed. Photodimerization proceeds efficiently with SE intercalated in the clay in the aggregated form. Overall photodecoloration, however, proceeds more efficiently with SB adsorbed to PVSK under multiple adsorption conditions apparently due to the higher mobility of PVSK-adsorbed SB compared to that of clay-intercalated SB.