Peroxynitric acid, HOONO2, can be prepared in aqueous solution by the reaction of nitrous acid with a greater-than-2-fold excess of hydrogen peroxide at or below 0 degrees C. Under optimum conditions, concentrations in excess of 1.5 M can be obtained. In acid solution at room temperature, the compound has a ca. 30 min half-life for decomposition; in alkaline solution, decomposition is very rapid, yielding nitrite ion and oxygen. The aqueous acid is a potent oxidizing agent, reacting rapidly with I-, Br-, Cl-, N-3(-), and VO2+. Raman, UV, and N-15 NMR spectra of the aqueous acid have been obtained. The Raman spectra reveal three new fundamental vibrations of the peroxynitric acid molecule, leaving only two that have yet to be positively identified. The UV spectrum shows a monotonic rise in absorbance between 290 and 230 Mn; the N-15 NMR resonance has a chemical shift of -28.3 ppm relative to dilute nitric acid.