Total atomization energies and enthalpies of formation (0 and 298.15 K) are evaluated using the high-accuracy extrapolated ab initio thermochemistry (HEAT) scheme for the two stable singlet C3H2 carbenes [cyclopro-penylidene (c-C3H2) and propadienylidene (vinylidencarbene, l-C3H2)], as well as for the 2-propynyl (propargyl, C3H3) radical. In the case of propargyl, the HEAT protocol predicts an enthalpy of formation of 354.9 +/- 1.0 kJ mol(-1) for 0 K; corresponding values of 498.1 +/- 1.0 and 555.6 +/- 1.0 U mol(-1) are estimated for c-C3H2 and I-C31-12. Additional consideration of temperature corrections leads to estimates of 352.2 +/- 1.0, 497.1 +/- 1.0, and 556.7 +/- 1.0 kJ mol(-1) for the heats of formation at 298.15 K of the propargyl radical, c-C3H2, and l-C3H2, respectively. Potential strategies for simplifying the HEAT protocol are also investigated and shown to have negligible impact on accuracy.