[en] The gaseous molecules VC, NbC, NbC₂, MoC, and MoC₂ have been identified in a mass spectrometric study of the Knudsen effusate from the niobium--graphite and vanadium--niobium--molybdenum--graphite systems at high temperatures. Third-law enthalpies of the various reactions involving these species were determined from the experimental equilibrium measurements and estimated thermodynamic functions. The reaction enthalpies in conjunction with published thermodynamic data yielded the thermochemical values (kJ mol^-1, kcal mol^-1): D⁰₀(VC)=419 +- 24, 101.1 +- 5.7; ΔH⁰/sub at,0/(VC₂)=1178 +- 17, 281.1 +- 4.1; D⁰₀(NbC)=564 +- 13, 134.8 +- 3.1; ΔH⁰/sub at,0/(NbC₂)= 1257 +- 13, 300.4 +- 3.1; D⁰₀(MoC)=478 +- 16, 114.2 +- 3.8; and ΔH⁰/sub at,0/(MoC₂)=1101 +- 20, 263.1 +- 4.8. In addition, the bond energies D⁰₀(V--C₂)=572 +- 19 kJ mol^-1, or 136.6 +- 4.6 kcal mol^-1; D⁰₀(Nb--C₂)=650 +- 16 kJ mol^-1, or 155.4 +- 3.8 kcal mol^-1; and D⁰₀(Mo--C₂)=494 +- 22 kJ mol^-1, or 118.1 +- 5.4 kcal mol^-1 were also calculated. The results are discussed in relation to the stabilities of previously known gaseous mono- and dicarbides of transition metal