A detailed quantum mechanical study of aldehydes and ketones has been carried out at the HF/6-31G* level. Computed relative conformational energies, rotational barriers, and geometries were calculated for a wide variety of molecules and compared with experiment. For the most part, both the computed relative conformational energies and the barriers are in reasonable agreement with experiment. In several cases there were differences observed between the quantum calculations and experiment which suggested a reinterpretation of the experimental data. For example, in the case of diisopropyl ketone, it was suggested that two conformers rather than the three assumed by experiment were present in equilibrium in the gas phase. For both cyclobutanecarboxaldehyde and methylcyclobutyl ketone the calculations predicted an additional (axial, gauche) low-energy minimum which has not been observed experimentally but should be possible to detect in the microwave spectrum. Relative to experiment, the computed C=O bond lengths are similar to 0.025 Angstrom smaller and the computed C=O stretches are similar to 280 cm(-1) higher. For cycloalkanones the calculations qualitatively reproduce the experimentally observed variation in the C=O bond length and the dramatic decrease in the vibrational frequency with increasing ring size. The dipole moments computed for aldehydes and ketones are similar to 10% higher than experiment with the exception of equatorial,trans-cyclobutanecarboxaldehyde, where it is 59% higher.