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(OCH 3 ) 2 (CH 3 )] 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.