Light scattering measurement, video-enhanced differential interference microscopic and electron microscopic observations, H-1-NMR measurement, and photocyclodimerization were carried out for aqueous solutions of alkyldimethylamine oxides (C(12)DAO and C(16)DAO) and dioctadecyldimethylammonium chloride (2C(18)DAC) with addition of cinnamic acid. Molecular assemblies in C(n)DAO:cinnamic acid solutions were converted from micelles to vesicles with increasing mixing ratios of cinnamic acid, while 2C(18)DAC in water always formed vesicles with addition of cinnamic acid. Structural changes of molecular assemblies constructed by amphiphilic compounds were reflected in the reactivity and stereochemical selectivity of photocyclodimerization. It was concluded that the photodimerization proceeded effectively in vesicles rather than in micelles. The dimerization yields increased with mixing ratio and were in the order of syn-HT < syn-HH < anti-HH (where HT and HH represent head-to-rail and head-to-head, respectively). The product selectivity was discussed in relation to the arrangement of cinnamic acid on molecular assemblies.