We report on the measurement of methyl radical(CH3)densities in a low-density supersonic arcjet plasma used in the synthesis of diamond films. Single-pass, high-sensitivity ultraviolet (UV) absorption spectroscopy has been employed to study theX(2A2″)→B(2A1′)transition of the methyl radical near 216 nm. The minimum detectableCH3density is found to be∼4×1013 cm−3, which corresponds to a fractional absorption of2×10−3at a gas temperature of 1200 K. The dependence of the measured methyl column density on pressure andCH4/H2flow ratio has been studied. The results are used to revise our previous estimates of the reactive “sticking” coefficient forCH3, and we now find that it is of order10−2under conditions where we have previously documented the growth of high quality diamond films. ©1997 American Institute of Physics.