The peroxynitrite anion, of vast importance in biochemistry, is formed in vivo from the reaction of NO and O-2(-). Laser ablation of 10 different metal targets with concurrent 7 K codeposition of NO/Ar and O-2/Ar mixtures gives new metal-independent infrared bands at 1458.3 and 806.1 cm(-1), and at 1433.3 and 983.2 cm(-1). in addition to known O-4(-) and (NO)(2)(-) absorptions. The new bands are not observed with CCl4 added to capture electrons or in O-2 and NO experiments without laser ablation to produce electrons, which identifies new product anions. Based on (NO)-N-15 and O-18(2) isotopic shifts, splitting patterns in mixed isotopic experiments, and comparison with DFT isotopic frequency calculations, the former absorptions are assigned to cis-OONO-, and the latter pair to trans-OONO-, which are isolated from metal cations trapped elsewhere in the matrix. The cis- and trans-peroxynitrite anion isomers are probably formed via the ion-molecule reaction between O-2(-) and NO: the O-2(-) anion, made by the capture of ablated electrons, is attested by the observation Of O-4(-). cis- and trans-OONO- are reversibly photoisomerized by visible and near-UV radiation. Collisional stabilization of the OONO-ion-molecule dimer complex during formation of the solid argon matrix appears to be crucial.