Two empirical methods have been developed to estimate the activation energy (E-a) of hydrogen-abstraction reactions involving hydrocarbons by using ground-state thermochemical properties: reaction enthalpy (DeltaH), broken bond energy (D-b), and formed bond energy (D-f). E-a = 12.67 + 2.98V(c) + 0.50DeltaH + 0.00604DeltaH(2) kcal/mol (method I); E-a = 54.47 + 4.21V(c) - 1.00D(f) + 0.00581D(b)D(f) + 0.00681(D-b - D-f)(2) kcal/mol (method II). The development of the methods is based on a fundamental understanding of the transition state structures (TSSs) and multiple regression analysis of a test set of 71 hydrogen-abstraction reactions involving the abstraction of alkane, allylic, and benzylic hydrogens. There is a significant effect of the pi-conjugate TSS on the correlation, which is discussed in terms of quantum chemical understanding. The effect of the pi-conjugate TSS is considered by adding an indicator variable V-c into the methods. The average absolute error and standard error are 0.47 and 0.64 kcal/mol for method I and 0.40 and 0.56 kcal/mol for method II for the test set. A comparison of the two proposed methods with previous empirical methods was performed statistically. The results show that the two developed methods significantly improve the estimation accuracy relative to the previous empirical methods.