The measured parameters for the formation of peroxynitrous acid via the reaction of acidified hydrogen peroxide with nitrous acid and its self-decomposition corroborate with an earlier suggested mechanism in which H2NO2+ nitrosates H2O2. The activation energies for the formation and decay of peroxynitrous acid have been determined to be 15 and 19 kcal/mol, respectively. We found that perchlorate, nitrate, sulfate and phosphate ions have no effect on the formation and decay rates, whereas chloride ions enhance the rate of the formation of peroxynitrous acid at low peroxide concentrations, and have no effect at high peroxide concentrations. This suggests that at relatively low concentration of H2O2, Cl- competes with H2O2 for H2NO+ to yield NOCl, which may also nitrosate H2O2. Simulation of the experimentally observed parameters for the decay and formation rates suggests that it is not possible to obtain 100% yield of peroxynitrite under any condition. High yields of peroxynitrite were obtained at room temperature using an efficient double mixer where acidified peroxide was mixed with nitrite; after an appropriate delay, the reaction was quenched with strong alkali. An excess of more than 10% of H2O2 over nitrite, or vice versa, is sufficient to get ca. 85-90% of peroxynitrite, almost free from nitrite or H2O2, respectively. The results also suggest that conventional use of ice-cold solutions of the reactants and the alkali solutions is not required if an efficient mixer and appropriate quenching times are available.