Absorption spectroscopy measurements are reported of the absolute number density (3.0 +/- 0.9 x 10(12) cm(-3)) and vibrational temperature (3000 +/- 500 K) of C-2((XEg+)-E-1) radicals in a DC arc activated chemical vapour deposition reactor under the standard operating conditions (3.3% CH4 in H-2 and 6.4 kW discharge) used to grow polycrystalline diamond films. The ratio of number densities of C-2 radicals in their ground (X(1)Sigma(g)(+)) and low-lying electronically excited (a(3)Pi(u)) states is close to that expected from a Boltzmann distribution at the measured temperature, despite the more rapid rates of reaction Of C2(X) with H2 and hydrocarbon molecules. (C) 2003 Elsevier B.V. All rights reserved.