We have calculated enthalpies of hydrogenation (Delta(hyd)H) and enthalpies of isomerization (Delta(isom)H) at 0 K and 298 K for 20 reactions involving C-4 hydrocarbons using the G2 and G2(MP2) nb initio methods. The mean unsigned difference between G2 and G2(MP2) values of Delta(hyd)H(298) is 0.13 kcal mol(-1), and the largest difference is 0.3 kcal mol(-1). The mean unsigned differences between G2 and G2(MP2) calculations and the five experimental results that are known for these reactions are 0.9 and 0.8 kcal mol(-1), respectively. Combination of Delta(hyd)H(298) Or Delta(isom)H(298) With a known enthalpy of formation Delta(f)H(298) in the set leads to the Delta(f)H(298) of any other molecule in the set. We have obtained values of Delta(f)H(298) = 126.6, 100.5, and 94.0 kcal mol(-1) for tetrahedrane, cyclobuta-1,3-diene, and methylenecyclopropene, respectively, and a computed Delta(isom)H of cyclobutane to methylcyclopropane of only -0.4 kcal mol(-1). Our results encourage use of the simpler G2(MP2) procedure for calculating Delta(f)H(298).