An investigation of the inhibition properties of Phosphorus-Containing Compounds (PCCs) in moderately strained (global strain rate of 300 s(-1)) non-premixed methane-N-2/O-2/Ar flames is presented. The effect of DMMP [dimethyl methylphosphonate, O=P(OCH3)(2)(CH3)] on relative OH concentration profiles was measured by using quenching-corrected Laser-Induced Fluorescence (LIF) for the first time. LIF measurements indicate a reduction in the total OH present of 23% for a non-premixed methane-air flame doped with 572 ppm of DMMP. As the stoichiometric adiabatic flame temperature is increased via substitution of Ar for N-2 in the oxidizer stream, the measurements show a strong decrease in the magnitude of the OH reduction. Experimental results show reasonable agreement with computational predictions made using a kinetic model that has been proposed for DMMP decomposition and phosphorus-radical chemistry. Analysis of the computational results shows that the reactions involving phosphorus remove H and O atoms from the radical pool, thus weakening the flame. These reactions produce OH directly, but the rest of the mechanism responds to O and H reductions by reducing OH levels. The key reactions involved in this inhibition process are identified. (C) 2001 by The Combustion Institute.