The improved G-L method was developed for determining the structural group composition of petroleum and coal tar fractions by using experimental values of refraction index, density, molecular weight, and S, N, O, and olefinic group content. The method is useful for fractions boiling in the range 30-500-degrees-C containing S, N, O and in total up to 10%, not limiting the distribution of the carbon atoms between aromatic, naphthenic, and paraffinic structures. Several correlations are proposed for prediction of the thermodynamic properties of petroleum and coal tar fractions, i.e., molar volume; surface tension; heat capacity in gas, liquid, and solid phases as a function of temperature; and also critical properties, standard heat and entropy of formation, and temperature and entropy of melting. The method and these correlations have been tested on hydrocarbons and other organic compounds with satisfactory accuracy.