Absolute line-of-sight averaged measurements of methyl radical concentrations in a microwave plasma-assisted diamond deposition reactor ave presented. The measurements are based on the use of broadband ultraviolet absorption spectroscopy to characterize the distinguishing absorption feature of methyl at 216nm associated with the X((2)A"(2)) --> B((2)A＇(1)) electronic transition. The dependence of the line-of-sight methyl concentration and mole fractions with the percentage of methane in the feed-gas, plasma power density, and position of substrate relative to the optical probe volume is studied The measurements suggest that the near-substrate methyl mole fraction is only weakly sensitive to changes in substrate temperature and are largely influenced by the gas phase temperature. A comparison is made between the measured mole actions and recent predictions based on a one-dimensional model of this process. The measured mole fractions are consistently greater than those predicted by about a factor of ten. This discrepancy is explained in part by the line-of-sight limitations in experimental facility.