Optical and molecular beam sampling-threshold ionization mass-spectrometric (MB-TIMS) measurements have been applied to fast-flowing, low-pressure C2H2/O/H/O-2 reactive systems at temperatures in the range 296 to 511 K. At each temperature, the 308 nm OH(A-->X) emission intensities were found to be directly proportional to the product of O-2 and CH concentrations, demonstrating that the CH + O-2 --> OH(A) + CO reaction channel (1d) is the source of electronically excited hydroxyl radicals in C2H2/O/H/O-2 atomic flames. Calibration of the detection systems for [O-2], [CH], and [OH(A)] yields a room-temperature rate constant for the reaction channel CH + O-2 --> OH(A) + CO of k(1d)(296 K) = 8.0(-5)(+10) x 10(-14) cm(3) s(-1), while the determinations at higher temperatures give an activation energy for reaction 1d of 0.7(-0.7)(+1.3) kJ mol(-1) over the 296 to 511 K range.