Abstract A rate theory for the thermal decomposition by a single bond fission at the high pressure limit was derived from the conceptions that the equilibrium between the active molecule and the activated complex is adiabatic and that the zero point energy difference between the molecule and the complex (ΔEz\eweq) participates in the entropy term of the rate constant (kd∞) and is not related to the enthalpy term. It was also proposed a method calculating kd∞ by replacing the vibrational state sum with the entropy term. The values of kd∞ for methanes and ethanes were calculated using the present theory and the Gorin activated complex model. From good correspondences between the experimental and calculated values, the values of ΔHf,298°(CH3(g))=143.2, D0(CH3–CH3)=393.0, and D0(CH3–H)=449.1 kJ mol−1 were obtained for the standard heat of formation and the chemical bond dissociation energy. The apparent activation energy for kd∞ decreases drastically by increasing of the reaction temperature. This fact means that the estimation of thermodynamic values from the apparent activation energy is not adequate. It was also found that the vibrational temperature in the complex is considerably higher than the reaction temperature by the contribution of ΔEz\eweq mainly. This result denotes that the chemical equilibrium constant cannot be evaluated from the ratio of kd∞⁄kc∞, where kc∞ means the high pressure limiting rate constant of the radical-radical recombination reaction.