Some phosphorus-containing compounds (PCCs). such as dimethyl methy[ phosphonate (DMMP) and trimethyl phosphate. have previously been considered as potential flame inhibitors. To investigate the flame inhibition behavior of PCCs, we employ the laser-induced fluorescence (LIF) technique to quantitatively measure hydroxyl (OH) concentrations in CH4/N-2-air opposed diffusion flames, undoped and doped with 0.01% or 0.1% DMMP in either the oxidizer or fuel stream, respectively. The methane content in the fuel stream ranges from 25% to 40% by volume, and the global strain rate (SR) is set at 20 s(-1). The percent reduction in total integrated OH number density in DMMP-doped flames decreases with increasing methane content, as a result of higher flame temperatures. Experimental OH profiles for DMMP-doped flames are compared with predicted profiles from kinetic modeling. The predicted reduction in total integrated OH severely underpredicts the measured reduction, thus suggesting a need for further improvements in our understanding of the chemical kinetics controlling PCC inhibition.