The gas-phase enthalpy of interconversion (Delta H degrees) between the axial and equatorial conformations of 3-methylcyclohexanone (3-MCHO) is determined to be 1.55+/-0.12 kcal mol(-1). The relative conformational concentrations are probed at various equilibrium temperatures from 303 to 453 K by 2+1 resonance-enhanced multiphoton ionization (REMPI) spectroscopy. We report that under our expansion conditions the measured relative peak areas of the two conformers in the REMPI spectrum are proportional to their relative pre-expansion concentrations. Delta H degrees [axial/equatorial] is obtained from a van’t Hoff plot of the measured relative peak areas of the two conformers as a function of temperature. Comparisons are made to molecular mechanics and various ab initio molecular orbital calculations including G2(MP2) theory.