The ideal gas thermodynamic properties [H °−H °0, (G°−H °0)/T, (H °−H °0)/T,S °,C °p, &Dgr;Hf °, &Dgr;Gf °, and logKf] for ethylene and propylene in the temperature range 0 to 1500Kand at 1 atm have been calculated by the statistical thermodynamic method employing the most recent fundamental and molecular spectroscopic constants. The internal rational contributions to thermodynamic properties for propylene were generated based on an internal rotation partition function formed by summation of internal rotation energy levels. The energy levels were derived from the potential functionV(cm−1) =349.2(1−cos 3&Vthgr;)−6.5(1−cos 6&Vthgr;). The calculated heat capacities and entropies were compared with the available experimental values.