Photocyclodimerization of unsubstituted and p-methoxy substituted cinnamic acids incorporated in micelles, vesicles or microemulsions formed by dodecyl-(C(12)DAO) and hexadecyl-dimethylamine oxides (C(16)DAO) has been studied in water. The dimerization proceeds in the vesicle much more efficiently than in the micelles with preferable formation of head-to-head dimers. The photoreactivity and the stereochemistry of cyclodimers are affected by structural change of the molecular aggregates.