The geometries of transition states and adducts have been optimized in MP2/6-311++G** calculations on both the singlet and triplet potential energy surfaces for the reaction of NO3 with OH. Single point calculations have been made at the G2MP2 level. Reaction is shown to be most likely to occur on the singlet surface via an adduct whose structure is very similar to that of pernitric acid, HO2NO2. The formation of the adduct is exothermic, and the calculated enthalpy of activation is -8 kJ mol(-1). This adduct can dissociate to HO2 + NO2 directly or via rotational isomers. Reaction on the triplet surface is unlikely to be important because the formation of a triplet adduct has a large enthalpy of activation.