Photoreactions induced by UV-B (290-320 nm) irradiation were studied for 1-methyl-2(1H)-pyrimidinone and 1-methylcytosine monomers isolated in low-temperature inert gas matrixes. A Norrish type 1alpha-cleavage reaction leading to open-ring conjugated isocyanate was observed for 1-methyl-2(1H)-pyrimidinone. The structure of the photoproduct was identified by comparison of its experimental IR spectrum with the spectrum theoretically calculated at the DFT(B3LYP)/6-31++G(d,p) level. The main indication of isocyanate as a photoproduced species was the appearance in the infrared spectrum recorded after UV irradiation of a very strong band at a characteristic frequency of 2263 cm(-1). Observation of a new band at nearly the same frequency in the spectrum of UV-irradiated monomeric 1-methylcytosine may suggest that also in the case of this compound a conjugated isocyanate is photoproduced.