The enthalpies of formation of cis- and trans-1,2-diethylcyclopropane were determined by oxygen bomb calorimetry. The cis-diethyl isomer was 1.1 kcal/mol less stable than the trans isomer. Alkyl groups were found to stabilize a cyclopropane ring by the same amount as for a carbon-carbon double bond. The enthalpies of formation cis- and trans-bicyclo[6.1.0]nonane also were determined by oxygen bomb calorimetry. The two bicyclo[6.1.0]nonanes have essentially the same enthalpies of formation, in contrast to cis- and trans-cyclooctene. The introduction of a trigonal center makes the trans-bicyclononane 3 kcal/mol less stable than the cis isomer. Molecular mechanics calculations are reported for a series of cis- and trans-bicyclo[n.1.0]alkanes. In order to better estimate the strain energy of trans-bicyclo[4.1.0]heptane, the difference in energy between it and its cis isomer was calculated via ab initio molecular orbital theory.