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We demonstrate some of the merits and limitations of using multiphoton ionisation (MPI) spectroscopy, resonance enhanced at the two photon energy by the state, to detect H atoms within a hot Ðlament 2s1 ;2S 1@2 reactor used for diamond chemical vapour deposition (CVD). Subsequent analysis of the Doppler broadened lineshapes obtained in this way allows determination of spatially resolved, relative H atom number densities and gas temperatures. The e †ects of pressure and Ñow rate, Ðlament temperature and radial distance from H 2 the Ðlament on the relative H atom number densities and the gas temperature proÐles have each been investigated for the case of pure and, in some cases, in the presence of added The present Ðndings H 2 CH 4 . complement and extend previous measurements obtained using alternative in situ detection methods, and are generally consistent with current models of the gas phase chemistry prevailing in low power diamond CVD reactors. They also serve to reÐne the role of the hot Ðlament in the H atom production process, and to illustrate continuing ambiguities regarding the way in which the local relative H atom number densities vary with Ðlament temperature, with the presence of trace quantities of added hydrocarbon, and the interpretation of these dependencies.