The thermodynamic properties (H0−H00, (G0−H00)/T, (H0−H00)/T, S0, Cp0, ΔHf0, ΔGf0, and log Kf] for methanoic (formic) and ethanoic (acetic) acid monomers and dimers in the ideal gaseous state over the temperature range from 0 to 1500 K and 1 atm have been calculated by the statistical thermodynamic method using the most recent and reliable molecular and spectroscopic constants. The internal rotational contributions of −OH and −CH3 rotors to the thermodynamic properties were evaluated based on internal rotation partition functions formed by summation of calculated internal rotation energy levels. On an assumption that the vapor contains only monomers and dimers, the thermodynamic properties for the monomer-dimer equilibrium mixture of methanoic and ethanoic acids in ideal gaseous state were derived. The results are in agreement with available experimental data.