The standard enthalpy of formation, estimated by the group contribution method of Benson, and by semi-empirical molecular simulation methods has been compared with experimental data. Benson's method estimates hydrocarbon enthalpies satisfactorily. The Benson's estimates of enthalpies of free radicals are in general superior to the molecular simulation parameterization method (PM3) and are less accurate than the specialized computationally intensive PM3-family correlation (PM3-FC) method. The estimation of enthalpies for hydrocarbon radicals is substantially improved by a linear correlation of estimated values from semi-empirical molecular simulation method PM3 with experimental data. The radical enthalpies are now comparable to molecular simulated PM3-FC estimates. The hydrocarbon cations were divided into classes and estimated enthalpies found by adding Benson's enthalpy of radicals to the average ionization energy for the class. These predictions have an average absolute deviation of 8.6 kcal mol(-1). The proposed correlations effectively predict the standard enthalpy of formation of hydrocarbons, free radicals and carbocations. This methodology can be readily implemented in simulation programs to estimate thermochemical properties of hydrocarbons, free radicals and carbocations and to improve the design and optimization of hydroprocessing units reducing costly hydrogen consumption. (c) 2005 Elsevier B.V. All rights reserved.