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 C(3)H(2) carbenes [cyclopropenylidene (c-C(3)H(2)) and propadienylidene (vinylidencarbene, l-C(3)H(2))], as well as for the 2-propynyl (propargyl, C(3)H(3)) 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 kJ mol(-1) are estimated for c-C(3)H(2) and l-C(3)H(2). 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-C(3)H(2), and l-C(3)H(2), respectively. Potential strategies for simplifying the HEAT protocol are also investigated and shown to have negligible impact on accuracy.