C2H, C-3 and C3H radicals and the C4H2 molecule are measured by threshold ionization mass spectrometry in a remote Ar/C2H2 expanding thermal plasma used for deposition of hydrogenated amorphous carbon films. Radical densities are calibrated and their fluxes towards the substrate are compared to the film growth rate. C2H has marginal contribution to the growth due to its fast gas phase reaction with C2H2 resulting in a low C2H density. The C-3 and C3H resonantly stabilized radicals behave differently due to their ultra low reactivity with C2H2 and are proposed to be significant growth precursors in plasma processes involving C2H2. (C) 2004 Elsevier B.V. All rights reserved.