CO2-permselective ceramic–carbonate dual-phase membranes offer potential for uses in separators or membrane reactors for conversion of coal derived syngas to hydrogen with CO2 capture. Coal derived syngas contains H2S, but the effect of H2S on permeation properties of these dual-phase membranes was unclear. This study examines the stability of Ce0.8Sm0.2O1.9 (SDC)–Li2CO3/Na2CO3 membrane in H2S-containing gas. CO2 permeation flux of the SDC–carbonate membrane is stable in a H2/N2/CO2 mixture gas, but it decreases after the introduction of H2S. The extent of the CO2 permeation flux reduction increases with H2S concentration in the feed gas and run time. Exposure to H2S results in appearance of a tree-branch-like structure near the feed side of the SDC–carbonate membrane, and formation of Ce2O2S on the SDC phase. Such an exposure has negligible effect on the carbonate phase. The formation of Ce2O2S lowers the oxygen ionic conductivity of the SDC phase, reducing the CO2 permeance for the dual-phase membrane.