The proton affinity of peroxyacetic acid has been studied using ab initio methods. The most stable peroxyacetic acid structure forms a nearly planar five-membered ring in which the carbonyl oxygen atom is hydrogen bonded to the acidic hydrogen atom. The rotational constants, dipole moments, and harmonic vibrational frequencies obtained for this structure are in excellent agreement with available experimental data. Calculations on the protonated peroxyacetic acid system identified low energy structures associated with protonation at each of the three chemically distinguishable oxygen atom sites. Protonation at the carbonyl oxygen site leads to the lowest energy protomer with an optimized structure that also contains a five-membered ring formed by intramolecular hydrogen bonding. The relative energies and proton affinities have been determined at the QCISD and QCISD(T) levels of theory. Additional calculations performed with the ab initio model chemistries CBS-4, CBS-Q, CBS-APNO, and G2(MP2) are in good agreement with the QCISD(T)/6-311 + G(2df,2p) proton affinity. The CBS-Q prediction for the proton affinity of peroxyacetic acid, PA(CBS-Q)(0K) = 775.9 kJ mol(-1), differs by only 2.5 kJ mol(-1) from the proton affinity recently calculated for peroxyacetyl nitrate, PA(CBS-Q)(0K) = -773.4 kJ mol(-1), and outside the error limits of the experimental measurement attributed to the proton affinity recently calculated for peroxyacetyl nitrate, -798 +/- 12 kJ mol(-1).