The enthalpies of formation of pure liquid and gas-phase ethyl 3-oxobutanoate and ethyl Z-3-hydroxy2-butenoate are examined in the light of some 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 (61.6 +/- 2.2) and (54.7 +/- 1.3) kJ(.)mol(-1) at T = 298.15 K are measured for pure ethyl 3-oxobutanoate and ethyl Z-3-hydroxy-2-butenoate, respectively. These values can be contrasted to a value of (54.2 +/- 1.0) kJ(.)mol(-1) when the vaporization enthalpy is measured as a mixture of tautomers. The difference is attributed to an endothermic enthalpy of mixing term of (7.1 +/- 2.1) kJ(.)mol(-1), which destabilizes the mixture relative to the pure components. Using the existing liquid-phase enthalpy differences measured by NMR, the endothermic enthalpy of mixing term and the condensed phase enthalpy of formation of the equilibrium mixture, new enthalpies of formation for ethyl 3-oxobutanoate and ethyl Z-3-hydroxy-2-butenoate in both the gas, Delta(f)H degrees(m)(g) = (-578.2 +/- 3.3), (-593.0 +/- 2.7) kJ(.)mol(-1), respectively, and liquid phases, Delta fH degrees(m)(l) = (-639.8 +/- 2.4), (-647.7 +/- 2.4) kJ(.)mol(-1), respectively, are calculated. A comparison of the difference in gas phase enthalpy of formation of the two tautomers reproduces the enthalpy difference measured previously by gas-phase NMR studies.