Ab initio calculations for the proton affinity of peroxyacetyl nitrate (PAN) reveal the complicated protonation chemistry; associated with the unusual multifunctional structure of this molecule. Optimized molecular structures, relative energies, decomposition energies, and proton affinities have been determined for four chemically distinguishable PANH(+) protomers. Calculations performed at the CBS-Q level of theory predict that the most stable structure is the peracetic acid-nitronium ion complex, [(CHC)-C-3(O)OOH . . . NO2+] and yield PA(CBS-Q)(0K)(PAN) = -773.4 +/- 10 kJ mol(-1). This result disagrees with the experimental determination of PA(298 K)(PAN) = -798 +/- 12 kJ mol(-1) that was attributed to protonation of the carbonyl oxygen but agrees with the experimental and theoretical results for the proton affinities of the nitrates HONO2, CH3ONO2, C2H5ONO2, and HOONO2, where the most stable protomers an also of the form [ROH . . . NO2+].