The critical flame extinction strain rates of opposed-flow hydrocarbon diffusion flames in the presence of various chemically-passive fire suppressants were experimentally and computationally determined. The objective was to extend an earlier study of chemically-passive suppressant effectiveness at normal-atmosphere conditions to the EVA-atmosphere conditions used in spacecraft prior to external vehicular activity. Extinction strain rates were determined for fuel streams consisting of pure CH4, C2H6, C3H8 or C2H4 and oxidizer streams composed of air with 0-30% volume fractions of Ar, N2 or CO 2 as inert suppressants. For both normal and EVA atmospheres, the relative suppressant effectiveness increased in order from Ar to N2 to CO2. This ordering is consistent with the simple increase in specific heat produced by the suppressant concentration in a stoichiometric mixture of the fuel and oxidizer. The internal structure of C2H 6 flames with and without each suppressant in normal and EVA atmospheres was examined using OPPDIF calculations to identify the mechanisms that determine the relative effectiveness of each suppressant. At any given suppressant concentration, peak flame temperature and maximum concentrations of OH, H and O radicals are higher in the EVA atmosphere than in normal atmosphere, consistent with the higher oxygen concentration under EVA conditions. This suggests that similar correlations as have been found at normal atmosphere for critical flame extinction strain rates with these parameters may govern suppressant effectiveness in the EVA atmosphere, allowing effectiveness of other suppressants at other atmospheric preparations to be inferred from these results. Results to date suggest that, due to the higher oxygen concentrations in the EVA atmosphere, the critical flame extinction strain rates are slightly higher in the EVA atmosphere than in a normal atmosphere.