A new method based on group contribution additivity, and using Benson's second order groups, is proposed for the prediction of critical temperatures and enthalpies of vaporization of covalent compounds. Contributions for hydrocarbons and hydrocarbon derivatives containing oxygen, nitrogen, chlorine, bromine and/or sulphur, are given. Results are compared to predictions made using the most common existing first or second order group contribution methods. The overall precision for T-c predictions of 381 compounds is 5.8 K. compared to 23.6 K with the method of Joback and 9.2 K with the method of Constantinou. The precision for predicted Delta H-vap of 319 compounds, at 298 K and at the normal boiling point, is improved by a factor 2 when comparing to the results of the method of Svoboda. Furthermore, one single group decomposition may now be used for the computation of gas phase properties, T-c and Delta H-vap at any temperature lower than T-c, leading to liquid phase thermochemical functions with better precision and simplicity. (c) 2006 Elsevier B. V. All rights reserved.