High-sensitivity absorption spectroscopy using a multi-element detector was performed on a microwave plasma-assisted chemical vapour deposition (MPACVD) diamond growth facility. The absolute methyl radical column density and the line-of-sight average of the gas temperature were measured as functions of position in the discharge with input methane concentration 0-1%. The average absolute methyl radical density [CH 3 ] was nearly independent of position in the discharge and varied in the range (1-6)×10 13 cm −3 as the input CH 4 mole fraction varied in the range 0-1%. The gas temperature was measured using H 2 emission spectroscopy and was found to be 1200 ± 100 K independent of position throughout the discharge. An upper bound of 10 10 cm −3 was placed on the CH density, which gives an upper bound of 0.008 on the hydrogen dissociation ratio [H]/[H 2 ].