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 x 10(7) dm(3) mol(-1) s(-1) and a backward rate constant of 1.06 x 10(5) s(-1). Decay of the HNO2-NO2 adduct would most likely generate HNO3 and NO at a rate constant of 3.0 x 10(3) s(-1). (C) 2004 Elsevier B.V. All rights reserved.