Effects of chemically-passive fire suppressants on the critical strain rate required to extinguish hydrocarbon-air opposed-flow diffusion flames were investigated by combined use of experiments and computations. Extinction strain rates were determined for fuel streams consisting of pure CH4, C 2H6, C3H8 or C2H 4 and oxidizer streams composed of air with 0-30% volume fractions of Ar, N2 or CO2 as inert suppressants. Relative suppressant effectiveness increased for all fuels in order from Ar to N2 to CO2, consistent with the increase in specific heat due to the resulting suppressant concentration in a stoichiometric mixture of the fuel and oxidizer streams. The higher suppressant effectiveness in the C 2H4 flames relative to previous measurements in wet-CO flames reflects the role of peak H-radical concentrations in the flame extinction process. Accordingly, although extinction strain rates for all fuels and suppressants correlate well with peak temperature just below the extinction limit, better correlation is found with the peak value of H radical concentration and with corresponding H+OH and H+O radical concentrations. However in contrast to suppressant effects in premixed flames, the present results do not indicate that the chain-terminating three-body recombination reaction H+O2+M → HO2+M controls the suppressant effectiveness in diffusion flames.