A highly sensitive multielement optical-absorption method is used to measure the absolute column densities of both CH₃ and CH in a d.c. plasma-assisted diamond chemical-vapor-deposition (CVD) system with an input gas of CH₄ diluted in H₂. The CH₃ column density decreases slowly with distance from the cathode and increases with increasing CH₄ input fraction. The CH density is sharply peaked near the cathode. The CH₃ and CH densities are compared with measurements of the same radicals made in a hot-filament diamond CVD system.