Because of the interest in their possible formation in the atmosphere, the conformational potential energy surfaces of ONOOH (peroxynitrous acid) and ONOOCl (chlorine peroxynitrite) were thoroughly investigated using correlated methods of electronic structure theory. Three conformers of ONOOH were found to be minima, the lowest in energy a planar five-membered hydrogen-bonded ring, predicted to lie 1 kcal mol(-1) below the form with a cis ONOO arrangement and perpendicular OH bond. Although predicted to be 3 kcal mol(-1) higher in energy than the planar conformer (in the gas phase), the identity of the form previously observed in matrix isolation experiments is suggested to have a trans ONOO arrangement and perpendicular OH bond, consistent with the experimental assignment. The chlorine analog ONOOCl was found to have two minimum energy conformers, the cis-perpendicular form predicted to lie 2 kcal mol(-1) below the trans-perpendicular form. Rotation about the peroxide bond is much more hindered in ONOOCl than in ONOOH. Calculations of physical properties, including geometrical structures and vibrational spectra; were made at the MP2 level of theory using basis sets ranging in quality from 6-31G(d) to 6-311G(2df,2p). Thermochemical tabulations based on G2 and G2(MP2) energies were made for ONOOH and ONOOCl, respectively. Values of Delta G(300 K), for production of the lowest energy conformers from reactants XO + NO2, are predicted to be -12 kcal mol-1 (X=H) and +3 kcal mol(-1) (X=Cl). The conformers of ONOOH may be formed by reactions in the atmosphere of HO NO2, while formation of the conformers of ONOOCl by reactions of ClO with NO2 is limited by unfavorable thermodynamics.