The HONO/SO2/N-2 and HONO/SO2/Ar matrixes were subjected to UV radiation (lambda > 340 nm) from a medium pressure mercury lamp. The products of the photolysis were studied experimentally by means of FTIR spectroscopy and theoretically by the ab initio MP2 method. Two species are formed in nitrogen matrixes: the hydroxysulfonyl radical HOSO2 and identified for the first time nitrososulfonic acid HO(NO)SO2. In solid argon only, the latter species was observed. The identities of the products have been confirmed by carrying out experiments with deuterated nitrous acid DONO in place of HONO, performing concentration and annealing studies, comparison with results of quantum chemical calculations, and reference to the spectral data of related molecules. The mechanism of the photolytic reaction in the studied HONO/SO2/N-2 and HONO/ SO2/Ar matrixes is discussed on the basis of the observed kinetic profiles of the precursor species and the products.