We have calculated the enthalpies of formation Delta H-f298 of cyclopropatriene, cyclopropadiene, cyclopropene, cyclopropane, allene, and propyne by the G2 method of Pople and co-workers. We present them here in conjunction with recently published G2 values of Delta H-f298 for propene and propane. The values of Delta H-f298 for all eight compounds are 180.6, 120.8, 68.6, 13.2, 45.5, 44.9, 5.3, and -25.4 kcal mol(-1), respectively. These results are discussed in light of experimentally measured Delta H-f298 results, drawn from enthalpies of combustion and of hydrogenation, for all but the first two of the compounds named.