The ionization energies (IEs) and heats of formation (Delta H degrees(f0)/Delta H degrees(f298)) for thiophene (C4H4S), furan (C4H4O), pyrrole (C4H4NH), 1,3-cydopentadiene (C4H4CH2), and borole (C4H4BH) have been calculated by the wave function-based ab initio CCSD(T)/CBS approach, which involves the approximation to the complete basis set (CBS) limit at the coupled-cluster level with single and double excitations plus a quasi-perturbative triple excitation [CCSD(T)]. Where appropriate, the zero-point vibrational energy correction (ZPVE), the core valence electronic correction (CV), and the scalar relativistic effect (SR) are included in these calculations. The respective CCSD(T)/CBS predictions for C4H4S, C4H4O, C4H4NH, and C4H4CH2, being 8.888, 8.897, 8.222, and 8.582 eV, are in excellent agreement with the experimental values obtained from previous photoelectron and photoion measurements. The Delta H degrees(f0)/Delta H degrees(f298) values for the aforementioned molecules and their corresponding cations have also been predicted by the CCSD(T)/CBS method, and the results are compared with the available experimental data. The comparisons between the CCSD(T)/CBS predictions and the experimental values for C4H4S, C4H4O, C4H4NH, and C4H4CH2 suggest that the CCSD(T)/CBS procedure is capable of predicting reliable IE values for five-membered-ring molecules with an uncertainty of +/-13 meV. In view of the excellent agreements between the CCSD(T)/CBS predictions and the experimental values for C4H4S, C4H4O, C4H4NH, and C4H4CH2, the similar CCSD(T)/CBS IE and Delta H degrees(f0)/Delta H degrees(f298) predictions for C4H4BH+ whose thermochemical data are not readily available due to its reactive nature, should constitute a reliable data set. The CCSD(T)/CBS IE (C4H4BH) value is 8.868 eV, and Delta H degrees(f0)/Delta H degrees(f298) values for C4H4BH+ and C4H4BH+ are 269.5/258.6 and 1125.1/1114.6 kJ/mol, respectively. The highest occupied molecular orbitals (HOMO) of C4H4S, C4H4O, C4H4NH, C4H4CH2, and C4H4BH have also been studied by the natural bond orbital (NBO) method, and the extent of a-electron delocalization in these five-membered rings are discussed in correlation with their molecular structures and orbitals.