The potential energy surfaces of all eight D-aldohexoses and four D-ketohexoses have been extensively studied, employing quantum mechanical and molecular mechanical calculations. Anomeric preferences for the axial OH positions were observed for all of the hexoses studied. Several stability factors determining the potential energy surface were examined, and we found that the Hassel-Ottar effect, the delta-two effect and the cis-trans effect are not discernible on the gas-phase potential energy surfaces of the cyclic hexoses. Instead, the anomeric effect and intramolecular hydrogen bonding effects dominate. The most important finding in this study is that in the gas phase. furanose forms are more stable than pyranose forms for all of the hexoses except galactose, idose, psicose, and sorbose, in contrast to the generally greater stability of pyranose forms observed in solution. The decrease of intramolecular hydrogen bonding stabilization in solution was found to be primarily responsible for the composition differences between the gas and solution phases.