UV-induced transformations were studied for monomers of 6-azacytosine isolated in low-temperature Ar matrices. In contrast to cytosine, where the amino-hydroxy (AH) tautomer is the lowest-energy form, the amino-oxo (AO) and imino-oxo (IO) isomers of 6-azacytosine were found to be the most stable and most populated. Due to the high relative energy of the AH tautomer of 6-azacytosine, this form is not populated in low-temperature matrices after their formation and prior to any irradiation. Excitation of 6-azacytosine monomers with UV light from the 328-300 nm range led to structural transformations of AO and IO forms. The initially most populated AO tautomer was observed either to convert, in a phototautomeric reaction, into the AH product or to undergo photodecarbonylation to yield 4-amino-1,2,3-(2H)-triazole. The relative efficiencies of the two processes depend on the wavelength and on the pulsed or continuous-wave character of the UV light used for excitation. For the IO tautomer of 6-azacytosine, the excitation with UV 328-300 nm light induced the photoconversion of the initially more populated anti IO1 isomer into the syn IO2 form. This transformation was found to be partially photoreversible.