The results of a study aimed at improvement of the group-contribution methodology for estimation of thermodynamic properties of organic substances are reported. Specific weaknesses where particular group-contribution terms were unknown, or estimated because of lack of experimental data, are addressed by experimental studies of enthalpies of combustion in the condensed phase, vapor-pressure measurements, and differential scanning calorimetric (dsc) heat-capacity measurements. Ideal-gas enthalpies of formation of cyclohexene, phthalan (2,5-dihydrobenzo-3,4-furan), isoxazole, octylamine, dioctylamine, trioctylamine, phenyl isocyanate, and 1,4,5,6-tetrahydropyrimidine are reported. Two-phase (liquid + vapor) heat capacities were determined for phthalan, isoxazole, the three octylamines, and phenylisocyanate. Liquid-phase densities along the saturation line were measured for phthalan and isoxazole in the temperature range 298 K to 425 K. The critical temperature and critical density of octylamine were determined from the dsc results and a critical pressure derived from the fitting procedures. Fitting procedures were used to derive critical temperatures, critical pressures, and critical densities for cyclohexene (pressure and density only), phthalan, isoxazole, dioctylamine, and phenylisocyanate. Group-additivity parameters or ring-correction terms useful in the application of the Benson group-contribution correlations are derived.