We report here a direct measurement of the spatially resolved atomic hydrogen concentration profiles during hot‐filament‐assisted chemical‐vapor deposition (HFCVD) of diamond films. The ground‐state hydrogen (1s 2S1/2) atoms generated in this process are monitored by an optical four‐wave‐mixing technique, third‐harmonic generation (THG). For THG, a 364.6 nm dye laser beam is focused into the HFCVD reactor and the third‐harmonic radiation near resonant with the Lyman‐&agr; (2p 2P0j↔1s 2S1/2) transition in atomic hydrogen at 121.6 nm is observed. The resultant THG intensity and THG peak shift with respect to the Lyman‐&agr; transition are both dependent on hydrogen atom concentration. Titration experiments based on the reaction NOCl+H→HCl+NO were conducted to obtain absolute hydrogen atom concentrations from the relative concentrations determined in the THG experiment. Spatially resolved molecular hydrogen temperature and concentration profiles obtained by coherent anti‐Stokes Raman scattering in a similar HFCVD reactor are reported. The observed H atom concentrations exceed computed equilibrium concentrations based on the measured gas temperatures. Transport of the atomic hydrogen from the hot filament surfaces is discussed and diffusion is shown to be the principal mechanism controlling the H atom distribution.