The molecular area of an amphiphilic azobenzene (AZ) bearing a carboxylic group was larger on aqueous subphase containing polyallylamine (PAA) than on the subphase containing Cd2+. In AZ/PAA LB films, AZ was deprotonated and PAA was protonated, indicating the salt formation of the two components. When the AZ/PAA LB films were stored in the dark, the absorption spectra of the LB films changed due to the variations in the orientation and the electronic structure of AZ. The LB films of AZ/Cd2+ showed similar structural changes and, in the final state, took almost the same structures with those of AZ/PAA LB films although the initial structures were different, AZ showed reversible photoisomerization in the AZ/PAA LB Films both in the initial and the saturated state. Photoisomerization of AZ did not proceed in the AZ/Cd2+ LB films either of the initial and the saturated state probably due to the lack in free volume.