Nitric acid (HONO2) in solid argon at 12 K was irradiated with ultraviolet light from various sources. Recombination of the fragments OH and NO2 from photolysis within the argon lattice site has led to the formation of peroxynitrous acid (HOONO). IR absorption lines at 3545.5, 1703.6, 1364.4, 952.0, and 772.8 cm-1 have been assigned to this molecule on the basis of isotopic shifts. Under certain conditions the lines at 3563.3, 1708.3, 1372.7, 957.4, and 782.9 cm-1 were also observed, and they have been attributed to HOONO in a less stable matrix site. The observed vibrational frequencies are in agreement with recent theoretical calculations on HOONO. The implication of the formation of HOONO from HONO2 to atmospheric chemistry is also discussed.