Experimental and modeling studies of the gas-phase chemistry occurring in dilute, hot filament (HF) activated B(2)H(6)/CH(4)/H(2) gas mixtures appropriate for growth of boron-doped diamond are reported. The results of two-dimensional modeling of heat and mass transfer processes and the B/H/C chemistry prevailing in such HF activated gas mixtures (supplemented by reactions involving trace O(2) present as air impurity in the process gas mixture) are discussed and compared with measurements of B atom densities as functions of the hot wire temperature T(w) and distance from the wire. Most of the B(2)H(6) molecules that diffuse from the cool, near-wall regions into the hot, near wire region are thermally decomposed (yielding two BH(3) molecules as primary products) and then converted into various 'active' B-containing species like B, BH and BH(2) - some of which are able to accommodate into the growing diamond film. H-shifting reactions BH(x) + H <-> BH(x-1) + H(2) enable rapid inter-conversion between the various BH(x) (x = 0-3) species and the BH(x) source is limited by diffusional transfer of B(2)H(6). H atoms play several key roles - e.g. activating the process gas mixture, and driving inter-conversions between the various H(x)B(y)C(z)O(z), species. We show that the T(w) and gas pressure dependences of the H atom production rate (by H(2) dissociation on the HF surface) can be accommodated by a simple gas-surface reaction model. (C) 2011 Elsevier B.V. All rights reserved.