The proton acceptor ability of a glycine dipeptide RO(O=)C-NH-CH2C(=O)-N(CH3)2 is investigated. Ab initio calculations are performed on N-methoxycarbonylglycine N',N'-dimethylamide (R = CH3) and compared with the experimental X-ray photoelectron spectrum of N-t-butoxycarbonylglycine N',N'-dimethylamide (R = C(CH3)3). The HF/6-3 IG* optimized structure shows that the dipeptide is stabilized by an intramolecular hydrogen bond between the NH and amide carbonyl groups. The molecular electrostatic potential and the atomic changes fitted to this potential are calculated. The HF/6-3 IG* 1s orbital energies of the carbon and oxygen atoms are computed and compared with the experimental X-ray photoelectron spectra. The difference between the proton affinities (PA) of the oxygen atoms can be extracted from these data. The PA of the urethane carbonyl oxygen is 26-29 kJ mol-1 higher than that of the amide carbonyl oxygen and 85-90 kJ mol-1 higher than that of the alcoxy oxygen. These results are compared with infrared data, which have shown that normal hydrogen bond formation with hydroxylic proton donors occurs at the oxygen atoms of the two carbonyl groups but that the urethane oxygen is the only protonation site.