Abstract The irradiation of barbital (BA) and N,N′-dimethylbarbital (DMBA) in aqueous alkaline media gave diethylacetylurea and N,N′-dimethyldiethylmalonarmide respectively. On the basis of the results of kinetic studies, the above photochemical hydrolyses of BA and DMBA were shown to be an attack of the hydroxide ion on the singlet excited states of BA monoanion and DMBA respectively. Such nucleophilic reagents as amines, mercaptide, and cyanide ions retarded the photochemical hydrolyses by quenching the excited state of the substrates.