Upon 2537A irradiation of uridylic acid a or b, an essentially quantitative conversion is obtained to an unstable product lacking the characteristic 260-m[mu] absorption band. The quantum yield for this conversion is 0.0216. The irradiation product can be isolated by ion exchange chromatography. In acid solution, or on heating of a neutral solution, the irradiation product reverts to the original uridylic acid. In alkaline solution the irradiation product is transformed to an unidentified substance.