Peroxynitrate (O2NOOH/O2NOO-) is formed within less than 2 ms after pulse irradiation of aerated solutions containing relatively low concentrations of formate and nitrate. The pK(a), for peroxynitric acid was determined to be 5.9 +/- 0.1 both from the pH-dependent absorbance of the anion at 310 nm and from the dependence of the decay kinetics on pH. An absorption spectrum was measured for the anion giving epsilon(max)(290)=1500+/-100 M-1 cm(-1). This method of generation of peroxynitrate is very useful for studying the mechanism of the oxidation of various substrates by peroxynitrate. The oxidation by peroxynitrate can take place either directly or indirectly. In the direct oxidation pathway, the reaction is first order in peroxynitrate and first order in the substrate, whereas in the indirect oxidation pathway, the reaction is zero order in the substrate. In both cases, the observed rate constants are highly pH-dependent. The results show that the direct oxidation pathway takes place through O2NOOH. We suggest that the indirect oxidation takes place through reactive intermediates that are formed during the decomposition of peroxynitrate. In the presence of sufficient concentrations of the substrates, the oxidation yields approach 100% through the direct and indirect oxidation pathways.