The relative inhibition efficacies of five different phosphorus agents are compared, including dimethyl methylphosphonate (DMMP), trimethyl phosphate (TMP), and three more complex but similar organophosphate compounds. Hydroxyl axial profiles are obtained for these inhibitors using a 30% CH4/70% N-2/air opposed diffusion flame, doped with 0.01 or 0.1 % inhibitor by volume in the oxidizer or fuel stream, respectively. An inverse correlation is found between the reduction in total integrated [OH] and the visual extinction concentration when the five phosphorus compounds are doped into the oxidizer stream. Hence, the present study demonstrates that the reduction in total integrated OH number density is a sensitive and effective parameter by which to investigate the inhibition capability of phosphorous-containing compounds. (C) 2003 The Combustion Institute. Published by Elsevier Inc. All rights reserved.