Spread monolayers of a photochromic trifluoromethoxy-substituted metacrylate copolymer were characterized by Pi/A isotherms under the condition of mainly trans or cis conformation of the photochromic side chains and during the trans restoration process. The isotherms of the individual components, i.e., trans and cis conformation, respectively, and those of variable trans/cis content were described by a new theoretical approach, which takes into account a possible side chain aggregation. A good agreement between the experimental Pi/A isotherms and the model was achieved for small aggregation numbers and a limited range of surface coverage. The trans/cis content during the trans recovery process was calculated by using the parameters of the individual components. The model correctly reflects the main characteristics of the individual components and the cis-trans transformation process: the minimum area demand per repeating unit of the trans conformation is smaller than that of the cis conformer; the aggregate tendency is higher for the trans conformers at low surface coverage; the trans/cis ratio increases during the trans restoration, where the content depends on the surface pressure.