Absolute concentrations of CH in a premixed, atmospheric flat flame of CH, and air have been determined with cavity ring-down spectroscopy (CRDS). CH is excited from the X(2)Pi to the A(2)Delta state at 430 nm. Since at atmospheric pressure the CH radical is present only in a very narrow layer at the flame front, specific problems due to the finite size of the laser beam and thermal deflection are encountered. An intensified CCD camera was used as an aid to be able to take these effects into account. Distributions of [CH] were obtained for two different stoichiometries (phi = 1.2 and 1.1) in a burner-stabilized flame. Signal-to-noise ratios indicate that [CH] number densities down to 4 X 10(11) cm(-3), corresponding to 1.5 ppb (S/N = 2) can be detected easily at I atm. Flame uniformity was verified with an Abel inversion technique. The rotational flame temperature was derived from Boltzmann distributions. The results were compared to modeling calculations using GRI-Mech 2.11 and 3.0. The predictions for both models show higher maximum [CH] located further away from the burner. The computed maximum [CH] is predicted in both cases at a higher temperature. Analyses of the effect of errors in the experimental settings and direct absorption measurements of [OH] have been used to verify positional differences. The results indicate that the differences may be attributed to reaction mechanisms. (C) 2003 The Combustion Institute. All rights reserved.