We study the suppressant effectiveness of five different gaseous agents: N-2, CO2, SO2, CF3Cl, and CF3Br. Laser-induced fluorescence is used to determine quantitative hydroxyl concentration profiles in opposed CH4/N-2-air diffusion flames with and without suppressants. Measurements are made by using a nitrogen-diluted counter-flow flame with 1% inhibitor added to the air stream. Visual extinction limits are also determined for each suppressant to determine the percent inhibitor in the air stream required for flame extinction. The suppression capabilities of all five agents are compared based on relating laser-induced fluorescence measurements of OH concentration to the visual extinction limits. An inverse correlation is found between the percent reduction in total integrated OH number density and the visual extinction limit. This correlation provides an effective protocol that can thus be used for studying and comparing different fire suppressants.