The enthalpies of formation of pure liquid and gas-phase (Z)-4-hydroxy-3-penten-2-one and 2,4-pentanedione are examined in the light of some more recent NMR studies on the enthalpy differences between gas-phase enthalpies of the two tautomers. Correlation gas chromatography experiments are used to evaluate the vaporization enthalpies of the pure tautomers. Values of (51.2 +/- 2.2) and (50.8 +/- 0.6) kJ(.)mol(-1) are measured for pure 2,4-pentanedione and (Z)-4-hydroxy-3-penten-2-one, respectively. The value of (50.8 0.6) kJ(.)mol(-1) can be contrasted to a value of (43.2 +/- 0.2) kJ(.)mot(-1) calculated for pure (Z)-4-hydroxy-3-penten-2-one when the vaporization enthalpy is measured in a mixture of tautomers. The difference is attributed to an endothermic enthalpy of mixing that destabilizes the mixture relative to the pure components. Calculation of new enthalpies of formation for (Z)-4-hydroxy-3-penten-2-one and 2,4-pentanedione in both the gas, Delta(f)H degrees(m)(g) = (-378.2 +/-1.2) and (-358.9 +/- 2.5) kJ(.)mol(-1), respectively, and liquid phases, Delta(f)H degrees(m)(g) = (-429.0 +/- 1.0) and (-410.1 +/- 1.2) kJ(.)mol(-1), respectively, results in enthalpy differences between the two tautomers both in the liquid and gas phases that are identical within experimental error, and in excellent agreement with recent gas-phase NMR studies.