The heat of formation and singlet-triplet splitting for cyanocarbene (HCCN) and isocyanocarbene (HCNC) have been determined in large-scale ab initio calculations, using the coupled-cluster method and basis sets of double- through quintuple-zeta quality. The heats of formation at 0 K for X(3)A'' HCCN and X(1)A' HCNC are predicted to be 116.8 and 140.5 kcal/mol, respectively, with the uncertainty estimated to be +/-2 kcal/mol or less. These values are substantially larger than those derived recently from experimental data, suggesting that the true values of the heats of formation must be at the upper end of their (large) experimental ranges. The singlet-triplet splittings are calculated to be 10.8 and 1.3 kcal/mol for HCCN and HCNC, respectively, with both values being in good agreement with the experimental data. The biradical nature of both molecules is discussed, on the basis of the results of calculations using the multireference methods.