The photochemistry of serum albumin has been studied in detail. Evidence has been obtained showing that approximately 5% of the radiation absorbed by the protein at 2537 angstrom wavelength is directly absorbed at the keto-imino linkage. It has been shown that irradiation of the protein in an atmosphere of nitrogen causes aggregation to give protein molecules of high molecular weight. Although there is some initial aggregation during irradiation in oxygen, this is quickly superseded by a photo-oxidative decomposition of the protein. The aggregation mechanism and kinetics have been elucidated. Absorption of radiation quanta by the keto-imino linkage causes fission of that linkage and the consequent production of two free radicals. These free radicals then react with the carboxyl and hydroxyl groups of other protein molecules forming aggregates. The free radicals have been detected, as has also their reaction with the carboxyl and hydroxyl groups. A theoretical equation for the rate of aggregation has been determined and shown to agree with that determined experimentally. It has also been possible to determine the quantum efficiency for the process in terms of the number of quanta absorbed by the peptide linkage. The photo-oxidative breakdown of the protein has been shown to be due to the formation in the solution of hydroxyl radicals. These react with the free radicals formed by the protein, thereby preventing aggregation.