A series of photoinduced electron-transfer fragmentation reactions have been studied in compressed monolayer films at the air/water interface. The reactions investigated involve amphiphilic and polymeric derivatives of fragmentable amino alcohol, 1,2-diamine, and pinacol donors and light-absorbing accepters, which are reactive in solution-phase studies from their triplet states. For intralayer studies a surfactant anthraquinone derivative was the light-absorbing acceptor. For comparable "interfacial" studies, the water soluble cation tris(2,2＇-bipyridine)ruthenium(II)(2+) (Ru(bpy)(3)(2+)) was the photoactive acceptor from the subphase. The fragmentation reactions all involve oxidative cleavage of a relatively strong C-C bond in the donor. Reaction was followed in each case by monitoring changes in surface pressure that occur when the compressed film is irradiated and maintained at a constant area. Reaction was readily observed in most cases where the donor and light-absorbing substrate are present; however the consequences were found to be quite dependent upon the specific donor substrate. Thus for simple single-chain amphiphiles containing either amino alcohol or 1,2-diamine donor sites, both intralayer and interfacial reactions result in rapid decrease in surface pressure, consistent with destruction of the film as the more hydrophilic redox products are solubilized into the subphase. For a polymeric diamine, much more complex behavior is observed, consistent with a situation where single fragmentation events do not lead to removal of material from the film but multiple fragmentation reactions culminate in film solubilization. Finally, a doublechain amphiphilic pinacol was found to undergo interfacial fragmentation with Ru(bpy)(3)(2+) in the subphase with a concurrent increase in surface pressure to form stable films that do not "dissolve" into the subphase. The isotherms observed following irradiation, decompression, and recompression are consistent with an expansion that occurs as the two-chain amphiphile undergoes redox fragmentation to produce two equivalents of a single-chain amphiphile.