The standard molar enthalpies of combustion, sublimation, and formation in the crystalline and gaseous state at a temperature of 298.15 K have been experimentally determined for the title compound (1) and for N-methylacetamide (2). These results have been combined with their gas-phase proton affinities to yield the standard enthalpies of formation of the species 1H+(g) and 2H+(g). These magnitudes have also been computed at the G2-(MP2) level. The agreement between experimental and theoretical results is very good. This information has been used to determine the strain in the neutral and protonated 6-lactam ring in the absence of solvent.