Peroxides have relatively low stability and are often difficult to obtain at needed purity for experimental studies. This study uses computational chemistry to estimate thermochemical properties of peroxides and compares the values to properties of corresponding hydroperoxides. The combined thermochemical properties are utilized to develop groups for use in group additivity. Structures, enthalpy (Delta(f)Hdegrees(298)), entropy (Sdegrees(298)), and heat capacity (C-p(T)) are determined for several vinyl, allyl, ethynyl, and phenyl peroxides using the density functional B3LYP/6-311G(d,p) calculations. Enthalpies of formation (Delta(f)Hdegrees(298)) are determined at the B3LYP/6-311G(d,p) level using four to six isodesmic working reactions. Entropy (S) and heat capacity (C-p(T)) values from vibration, translational, and external rotational contributions are calculated using the rigid-rotor-harmonic-oscillator approximation based on the vibration frequencies and structures obtained from the density functional studies. Contribution to Delta(f)H(T), S, and C-p(T) from analysis on the internal rotors is included. Delta(f)Hdegrees(298) for vinyl-methyl peroxide, CH2=CHOOCH3, is -10.04 +/- 0.66 kcal mol(-1) and for allyl-methyl peroxide, CH2=CHCH2OOCH3, is -12.08 +/- 1.05 kcal mol(-1). Methyl-substituted vinyl peroxide values are CH2 = C(CH3)OOCH3 = -20.79 +/- 0.42, CH3CH = C(CH3)OOCH3 = -29.58 +/- 0.32, and CH3(CH3)C = CHOOCH3 = -30.60 +/- 0.90 kcal mol(-1). The cis conformation of CH3CH = CHOOCH3, -21.27 +/- 0.47 kcal mol(-1), is more stable than the trans form, -19.95 +/- 0.67 kcal mol(-1). Enthalpies for ethynyl peroxides are 41.31 +/- 0.72 kcal mol(-1) for HCdropCOOCH(3) and 29.51 +/- 0.27 kcal mol(-1) for CH3C=COOCH3. The calculated Delta(f)Hdegrees(298) for phenyl peroxide, C6H5OOCH3, is -2.19 +/- 0.52 kcal mol(-1). The resulting peroxide enthalpies allow determination of the RO-OC, ROO-C bond energies. The vinyl, phenyl and ethynyl peroxides are unstable and rapidly dissociate via cleaving of the weak RO-OR' peroxide bond with formation of a strong carbonyl bond. Enthalpy of formation was also calculated for CH3OOCH3 (-30.77 +/- 0.64 kcal mol(-1)) and CH3CH2-OOCH3 (-39.0 +/- 0.24 kcal mol(-1)) due to the need for these enthalpy values in the working reaction analysis. Thermodynamic properties for the oxygenated carbon groups O/C-d/O, O/C-b/O, C-t/O, O/C-t/O, and O/C/C-t were also determined. The agreement between group additivity and calculated enthalpies and heat capacities for the different classes of molecules studied supports the group additivity principal for these systems.