An aq. soln. of peroxynitrous acid has been studied using first-principles mol. dynamics simulations based on d. functional theory. The relative Helmholtz energies of different conformers have been detd. via thermodn. integration with constraints. At contrast to the gas phase, only two conformers, a cis and a trans isomer, are present in soln. and their relative Helmholtz energy is enhanced with respect to the gas phase. The av. structural properties of the two conformational forms on the other hand are very close to the resp. gas phase values. The interconversion pathway between the two conformers has been detd., and the Helmholtz energy profile for the isomerization reaction in soln. is presented. The rotational barrier is calcd. to be substantially higher than in gas phase due to a strong rearrangement of the solvent during the reaction. The structure of the transition state can only be described correctly when the solvent is taken explicitly into account. Our calcns. indicate that the cis form is the dominant species in aq. soln. at ambient temps. [on SciFinder (R)]