The results of a study aimed at improvement of group-contribution methodology for estimation of enthalpies of formation in the ideal-gas state for pure 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, vaporpressure measurements, and differential scanning calorimetric (DSC) heat-capacity measurements. Enthalpies of formation in the condensed phase were determined for 1,4-diisopropylbenzene, 1,2,4,5-tetraisopropylbenzene, dimethyl malonate, glutaric acid, pimelic acid, and cyclohexanone oxime. Ideal-gas enthalpies of formation for each of the compounds except pimelic acid are also reported. Enthalpies of fusion and for crystalline phase transitions were determined for 1,2,4,5-tetraisopropylbenzene, cyclohexanone oxime, glutaric acid, and pimelic acid. Two-phase (solid + vapor) or (liquid + vapor) heat capacities were determined from 300 K to the critical region or earlier decomposition temperature for all the title compounds. For 1,4-diisopropylbenzene, 1,2,4,5-tetraisopropylbenzene, and dimethyl malonate, values of the critical temperature and critical density were determined from the DSC results and the corresponding critical pressure was derived from the fitting procedures. The results of all the measurements were combined to derive a series of thermophysical properties including critical temperature, critical density, critical pressure, acentric factor, enthalpies of vaporization (restricted to within :1:50 K of the temperature range of the vapor pressures), and heat capacities along the saturation line. Wagner-type vapor-pressure equations were derived for each compound. Enthalpies of sublimation were derived for cyclohexanone oxime and, using literature vapor pressures, glutaric acid. Group-additivity enthalpy of formation parameters and strain energies useful in the application of ideal-gas-phase group-contribution correlations were derived. Errors in the literature data for the enthalpy of formation of diethyl malonate are noted.