The isolation, analysis, and synthesis of the principal product of the ultraviolet (2537 Å) irradiation of aqueous 1,3-dimethyluracil has made it possible to establish that the irradiation results in hydration of the 5:6 double bond of the uracil ring to form 1,3-dimethyl-6-oxy-hydrouracil. This hydration is spontaneously reversible, the dehydration being catalyzed by either acid or base. Kinetic studies of the catalyzed regeneration reactions are reported