The high-temperature Knudsen effusion mass spectrometric method was used in measuring the equilibrium partial pressures of C-1-C-7 above pure graphite. These data were combined with new thermal functions for linear C-4-C-7, which were calculated from recent spectroscopic and theoretical data in evaluating enthalpies of formation for C-2-C-5. The following enthalpies of formation, Delta(f)H(C-n,g,0) and of atomization, Delta(a)H(C-n,g,0), in kJ mol(-1), were obtained on the basis of the third law method : C-2, 817 +/- 8 and 605 +/- 8; C-3, 831 +/- 13 and 1303 +/- 13; C-4, 1052 +/- 16 and 1793 +/- 17; and C-5 1081 +/- 16 and 2475 +/- 17. The enthalpies of formation for C-4 and C-5 are higher than previously believed. They are in good agreement with recent theoretical predictions.