Ab initio molecular orbital calculations were performed using the GAUSSIAN 92 system of programs at the RHF/6-31G* level of theory for 1,1,2-trifluoroethane (CH2FCHF2) and 1,1,2,2-tetrafluoroethane (CHF2CHF2). Equilibrium geometries, barriers for internal rotation, and harmonic vibrational frequencies were thereby determined. Electron correlation energy corrections were performed by single-point calculations at both second- and fourth-order Moller-Plesset perturbation theory. Two conformational minima in the potential energy surface were located for each molecule. The anti structure found in the CH2FCHF2 molecule was determined to be 1.66 kcal/mol more stable than the gauche structure at the MP4/6-311G**//6-31G* level after inclusion of zero-point vibrational energy correction. Similar calculations at the same level of theory determined that the trans structure for the CHF2CHF2 molecule was 1.73 kcal/mol more stable than the gauche conformer. Two rotational transition structures were located for each molecule. The barriers for rotation about the C-C bond at the MP4/6-311G** level were determined to be 2.80 and 5.61 kcal/mol for CH2FCHF2 and 3.90 and 7.11 kcal/mol for CHF2CHF2. Computed thermodynamic properties including heat capacity, entropy, enthalpy, and free energy functions are reported as a function of temperature. A theoretical heat of formation was evaluated for both molecules from applicable homodesmic reactions (Delta H degrees(f,298)(CH2FCHF2) = -156.8 +/- 1.3 kcal/mol and Delta H degrees(f,298)(CHF2CHF2) = -209.1 +/- 1.7 kcal/mol). These data were used to evaluate Delta H degrees(f,T), Delta G degrees(f,T), and K-f,K-T for both molecules over the temperature range of 0-1500 K.