The reaction between the hydroxyl radical and the nitrogen dioxide radical in the gas phase has been studied with ab initio mol. dynamics simulations based on d. functional theory. Nitric acid HONO2 and cis-cis peroxynitrous acid ONOOH are formed spontaneously, whereas for the formation of the corresponding trans-perp conformer of peroxynitrous acid, a barrier is obsd. The relative energetics of the different conformers of ONOOH are in good agreement with MP2 results. The same holds for the structural properties, except for the N-O single bond which turns out to be highly sensitive to the exchange-correlation functional and is generally overestimated. [on SciFinder (R)]