Oxidation of unsaturated and aromatic hydrocarbons in atmospheric and combustion processes results in formation of linear and cyclic unsaturated, oxygenated-hydrocarbon intermediates. The thermochemical parameters Delta(f)H(298)degrees, S(298)degrees, and C-pf298(T) for these intermediates are needed to understand their stability and parameters Delta(f)H298 degrees, S(298)degrees, and C-pf298(T) reaction paths in further oxidation. These properties are not available for a majority of these unsaturated oxy-hydrocarbons and their corresponding radicals, even via group additivity methods. Enthalpy, entropy, and heat capacity of a series of 40 oxygenated and non-oxygenated molecules, or radicals corresponding to hydrogen atom loss from the parent stable molecules are determined in this study. Enthalpy (Delta(f)H(298)degrees in kcal mol(-1)) is derived from the density function calculations at the B3LYP/6-311g(d,p) calculated enthalpy of reaction (Delta H(rxn,298)degrees) and by use of isodesmic (work) reactions. Estimation of error in enthalpy (Delta(f)H(298)degrees), from use of computational chemistry coupled with work reactions analysis, is presented using comparisons between the calculated and literature enthalpies of reaction. Entropies (S(298)degrees) and heat capacities (C-pf298(T)) were calculated using the B3LYP/6-311G(d,p) determined frequencies and geometries. Potential barriers for internal rotors in each molecule were determined and used (in place of torsion frequencies) to calculate contributions to S and C-p(T) from the hindered rotors. Twenty-six groups for use in group additivity (GA) are also developed.