An aqueous solution of peroxynitrous acid has been studied using first-principles molecular dynamics simulations based on density functional theory. The relative Helmholtz energies of different conformers have been determined via thermodynamic integration with constraints. At contrast to the gas phase, only two conformers, a cis and a trans isomer, are present in solution and their relative Helmholtz energy is enhanced with respect to the gas phase. The average structural properties of the two conformational forms on the other hand are very close to the respective gas phase values. The interconversion pathway between the two conformers has: been determined, and the Helmholtz energy profile for the isomerization reaction in solution is presented. The rotational barrier is calculated 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 calculations indicate that the cis form is the dominant species in aqueous solution at ambient temperatures.