Cyclopentane is conventionally strained. Replacement of a carbon atom by a heteroatom obviously impacts angular strain in the five-membered ring compounds. Changes of strains in the five-membered cycles are also caused by a double bond or atttached benzene rings. We studied the thermochemical properties of Indane, 2,3-dihydrobenzofuran, indoline, N-methyl-indoline, carbazole, and N-ethyl-carbazole to obtain a better quantitative understanding of the energetics associated with these compounds containing five-membered ring units. We used combustion calorimetry, transpiration method, and high-level first-principles calculations to derive gaseous enthalpies of formation of the five-membered heterocyclic compounds. Our new values together with the selected values for parent heterocyclic compounds, available from the literature, were used for calculation of the strain energies H(S) of five-membered C-, N-, and O-containing cycles. Quantitative analysis of the resulting stabilization or destabilization of a molecule due to interaction of benzene rings with the heteroatom has been performed.