Tantalum hot filaments (HFs) find frequent use as a means of activating hydrocarbon/hydrogen mixtures used for chemical vapour deposition of thin film diamond. This contribution reports systematic studies of the power consumed by a tantalum HF, and companion laser based measurements of the relative H atom number densities in the gas phase adjacent to the HF surface, in pure H-2 and in dilute CH4/H-2 gas mixtures, as a function of process conditions (filament temperature, gas pressure, extent of HF carburisation). The measurements serve to highlight the way in which the adjacent gas phase chemistry and composition affects the HF surface chemistry, and vice versa. (C) 2007 Elsevier B.V. All tights reserved.