AbstractThe energies of various conformations have been calculated by molecular mechanics for cis and trans isomers of 2,4,7,9‐tetraoxabicyclo[4.4.0] decane and 3,5,8,10‐tetraoxabicyclo[5.3.0]decane and their methyl derivatives. These molecules are models for reaction products from formaldehyde and the tetrols, pentitols, and hexitols. The conformational equilibria were analyzed for thecis‐bicyclo [4.4.0] andcis‐bicyclo[5.3.0] systems and compared with available experimental data. The thermodynamic stability of bicyclo[4.4.0]products was found to be higher than that of bicyclo[5.3.0] derivatives in the gas phase in every case studied. Discrepancies with experimental data that exist in a few cases can be ascribed to solvent