Thermochemical properties of indene, 2,3-benzofuran, indole, and N-methylindole have been studied to obtain a better quantitative understanding of the energetics associated with these compounds containing five-membered ring units. We used combustion calorimetry, transpiration, and high-level first-principles calculations to derive gaseous enthalpies of formation of the five-membered heterocyclic compounds. Our new values together with selected values for the parent heterocyclic compounds, available from the literature were used for calculation of the strain enthalpies H(S) of five-membered C, N, and O-containing cycles. The quantitative analysis of the resulting stabilization or destabilization of a molecule due to interaction of the benzene ring with the heteroatom has been performed.