Photochemical and thermal cis/trans isomerization is reported for a stilbazolium betaine (M). Obtained from synthesis in the trans configuration, M cannot be isomerized directly but only via the 0-protonated form MH&ns. Photoisomerization of MH;,,, yields the cis isomer MH& (structure established by IH NMR) which can be deprotonated to the cis betaine. M,i, represents the first example of a cis isomer in this class of compounds. Though fairly stable in aqueous solution, M,i, can be fully reverted to the trans isomer either thermally or photochemically. The sequence M,,,,, s MH:,,, F! MH;, ~i M,i, -M,,,,, constitutes a complete molecular reaction cycle which may serve as a chemical model for the storage of information and subsequent regeneration of the information carrier, for instance, in biological systems.