A combination of matrix isolation and Fourier transform infrared (FTIR) spectroscopy has been employed to investigate the low-temperature photochemistry (4.2 K) of nitric acid (HONO2). UV photolysis of the matrix-isolated precursor in argon (MR 1/4000) at 184.9 and 253.7 nm led to the formation of peroxynitrous acid (HOONO), presumably via cage recombination of OH + NO2. HOONO appeared to undergo secondary photolysis to yield NO and HO2. A second primary photolysis channel produced the novel species C-2v-HNO2, a thermodynamically less stable isomer of nitrous acid; deuterated studies combined with group frequency arguments and theoretical studies support this assignment. In nitrogen matrices, N2O, cis-HONO, and trans-HONO were observed as additional products due to efficient scavenging of O(D-1(2)) by the matrix material. Irradiation of HONO2 in oxygen-doped argon matrices resulted in photooxidation and produced primarily peroxynitric acid (HO2NO2) as well as ozone.