Abstract—Ultraviolet irradiation of orotic acid in aqueous medium leads to the formation of dimers with a cyclobutane ring between the 5,6 bonds of two monomers. These photodimers in turn undergo photodissociation, with the result that an equilibrium is established between photodimerization and photodissociation. The equilibrium point is independent of dose rate, but is appreciably dependent on the irradiation wavelength, pH of the medium (i.e. whether the monomer is in the neutral or anionic form), and initial monomer and oxygen concentrations; the effect of oxygen apparently being to deactivate excited monomers. The kinetics of photodimerization and photodissociation have been examined at different wavelengths and with varying oxygen concentrations at different initial monomer concentrations. The results have been compared with a theoretical model in which it is assumed that a dimer is formed between an excited monomer and second molecule in the ground state, and that oxygen can also react with, and deactivate, an excited monomer. The agreement between the proposed model and the experimental data is reasonably good, and has made possible an estimate of the lifetime of the excited state of orotic acid in aqueous medium, which is about 10‐6