The effects of atmosphere composition on the rate of opposed-flow flame spread (Sf) over thin solid fuel beds at microgravity (μg) were measured and compared to earth gravity (lg) 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 ug 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 μg than lg. Secondly, the effect of diluent type onSf-was studied by comparing results using He, N2, Ar, CO2and SF6diluents. It was found that, in agreement with prior studies in N2diluent, for He, N2or Ar diluents,Sfwas larger at 1g than μg. In contrast, for CO2diluent,Sfwas slightly lower at 1g than at μg and for SF6diluent,Sfwas much lower at 1g than μg. Moreover, unlike He, N2and Ar, for CO2and especially SF6diluents the minimum O2concentration required to support flame spread was lower at μg than 1g. For SF6, the minimum O2concentration at μg 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.