In present work, a complete study of thermodynamic properties of pyruvic acid and methyl pyruvate in the condensed and gaseous phases was carried out using experimental and theoretical approaches. Enthalpies of combustion and formation of compounds were evaluated using combustion calorimetry. The transpiration method was used to determine the temperature dependence of the vapor pressures of acid and its methyl ester for the vaporization enthalpy calculation. With ab initio calculation it has been determined, that in gas phase pyruvic acid exists predominantly in monomeric form. The values of the enthalpy characteristics of the substances obtained using the composite method G4 were checked for reliability. A good agreement between the calculated and experimental values of the enthalpies of formation of the acid and its methyl ester in the gaseous phase is observed. The methods of quantum chemistry and statistical physics were used to calculate thermodynamic functions of compounds in the ideal gas state in temperature range 298.15-1500 K.