The effect's of atmosphere composition on the rate of opposed-flow flame spread (S-f) over thin solid fuel beds at microgravity (mu g) were measured and compared to earth gravity (Ig) results and theoretical predictions. Two modifications to standard atmospheres were considered. First, the effects of sub-flammability-limit concentrations of a gaseous fuel (CO or CH4) were measured and compared to an existing theoretical model that was extended to mu g conditions. The agreement between the model and experiment is reasonable considering the simplicity of the model. Notably, both model and experiment show that the effect of added gaseous fuel is greater at mu g than Ig. Secondly, the effect of diluent type on S-f was studied by comparing results using He, N-2, Ar, CO2 and SF6 diluents. It was found that, in agreement with prior studies in N-2 diluent, for He, N-2 or Ar diluents, S-f was larger at Ig than mu g. In contrast, for CO2 diluent, S-f was slightly lower at Ig than at mu g and for SF6 diluent, S-f was much lower at Ig than mu g. Moreover, unlike He, N-2 and Ar, for CO2 and especially SF6 diluents the minimum O-2 concentration required to support flame spread was lower at mu g than Ig. For SF6, the minimum O-2 concentration at mu g was even lower than the upward (concurrent-flow) limit. This unusual behavior is proposed to be a result of reabsorption of radiation emitted from the gases, which is significant only for gases with small mean absorption lengths.