We have calculated the enthalpies of hydrogenation, isomerization, and formation in the gaseous state (Delta H-hyd,(298) Delta H-isom,(298) and Delta H-f(298)) Of cyclopentenes, bicyclo[3.3.0]octenes, and triquinacenes by the G3(MP2) procedure. Comparison with combustion and hydrogenation thermochemical results shows that agreement between G3(MP2) calculations and experiment remains well within the target interval of 1 to 2 kcal mol(-1) for those cases in which experimental results exist. The sole exception is our earlier study on the hydrogenation of triquinacene to perhydrotriquinacene:. which deviates from both calculation and a more recent combustion study.