The OH quantum yield from the photolysis of nitrous acid in the aqueous phase by the 355 nm light was measured to be 0.25 ± 0.03. OH radical thus formed reacted readily with HNO2 to produce NO2, which sequentially reacted with HNO2 to form the HNO2–NO2 adduct. The NO2 + HNO2 reaction was reversible with a forward rate constant of 3.76 × 107 dm3 mol−1 s−1 and a backward rate constant of 1.06 × 105 s−1. Decay of the HNO2–NO2 adduct would most likely generate HNO3 and NO at a rate constant of 3.0 × 103 s−1.