We report the ab initio G2(MP2) and G2(MP2,SVP) standard enthalpies of hydrogenation and isomerization (Delta(hyd)H and Delta(isom)H) at 298 K of 23 C-5 cyclic hydrocarbons, including methyl- and methylenecyclobutane, spiropentane, and related unsaturated hydrocarbons, as well as methyltetrahedrane, Delta(hyd)H(298) and Delta(isom)H(298) are found from the differences in total energy (E-0) of the participants in the hydrogenation and isomerization reactions. Combination of calculated Delta(hyd)H(298) and Delta(isom)H(298) with one experimental enthalpy of formation (Delta(f)H(298)) in the set yields Delta(f)H(298) of all the est. In those few instances for which reliable experimental results can be found, agreement between experiment and calculation is within 1 kcal mol(-1).