Heats of formation of the lowest triplet state of ethylene and the ground triplet state of ethylidene have been predicted by high level electronic structure calculations. Total atomization energies obtained from coupled-cluster CCSD(T) energies extrapolated to the complete basis set limit using correlation consistent basis sets (CBS), plus additional corrections predict the following heats of formation in kcal/mol: Delta H-f(0)(C2H4, (3)A(1)) = 80.1 at 0 K and 78.5 at 298 K, and Delta H-f(0)(CH3CH, (3)A '') = 86.8 at 0 K and 85.1 at 298 K, with an error of less than +/- 1.0 kcal/mol. The vertical and adiabatic singlet-triplet separation energies of ethylene were calculated as Delta E-S-T,E-vert = 104.1 and Delta E-S-T,E-dia = 65.8 kcal/mol. These results are in excellent agreement with recent quantum Monte Carlo (DMC) values of 103.5 +/- 0.3 and 66.4 +/- 0.3 kcal/mol. Both sets of computational values differ from the experimental estimate of 58 3 kcal/mol for the adiabatic splitting. The computed singlet-triplet gap at 0 K for acetylene is Delta E-S-T,E-adia(C2H2) = 90.5 kcal/mol, which is in notable disagreement with the experimental value of 82.6 kcal/mol. The heat of formation of the triplet is Delta H-f(0)(C2H2,B-3(2)) = 145.3 kcal/mol. There is a systematic underestimation of the singlet-triplet gaps in recent photodecomposition experiments by similar to 7 to 8 kcal/mol. For vinylidene, we predict Delta H-f(0)(H2CC,(1)A(1)) = 98.8 kcal/mol at 298 K (exptl. 100.3 +/- 4.0), Delta H-f(0),(H2CC, B-3(2)) = 146.2 at 298 K, and an energy gap Delta ES-T-adia(H2CC) = 47.7 kcal/mol.