Thermochemical properties for isooctane, its internal rotation conformers, and radicals with corresponding bond energies are determined by use of computational chemistry. Enthalpies of formation are determined using isodesmic reactions with B3LYP density function theory and composite CBS-QB3 methods. Application of group additivity with comparison to calculated values is illustrated. Entropy and heat capacities are determined using geometric parameters, internal rotor potentials, and frequencies from B3LYP/6-31G(d,p) calculations for the lowest energy conformer. Internal rotor potentials are determined for the isooctane parent and for the primary, secondary, and tertiary radicals in order to identify isomer energies. Intramolecular interactions are shown to have a significant effect on the enthalpy of formation of the isooctane parent and its radicals. The computed standard enthalpy of formation for the lowest energy conformers of isooctane from this study is -54.40 +/- 1.60 kcal mol(-1), which is 0.8 kcal mol(-1) lower than the evaluated experimental value -53.54 +/- 0.36 kcal mol(-1). The standard enthalpy of formation for the primary radical for a methyl on the quaternary carbon is -5.00 +/- 1.69 kcal mol(-1), for the primary radical on the tertiary carbon is -5.18 +/- 1.69 kcal mol(-1), for the secondary isooctane radical is -9.03 +/- 1.84 kcal mol(-1), and for the tertiary isooctane radical is -12.30 +/- 2.02 kcal mol(-1). Bond energy values for the isooctane radicals are 100.64 +/- 1.73, 100.46 +/- 1.73, 96.41 +/- 1.88 and 93.14 +/- 2.05 kcal mol(-1) for C3 center dot CCCC2, C3CCCC2 center dot, C3CC center dot CC2, and C3CCC center dot C2, respectively. Entropy and heat capacity values are reported for the lowest energy homologues.