A rubber composition comprising (A) an organopolysiloxane end-capped with alkenyldialkylsiloxy, dialkenylalkylsiloxy or trialkenylsiloxy radicals and free of aliphatic unsaturation on any side chain, (B) an organopolysiloxane containing at least three alkenyl radicals on molecular side chains, (C) an organohydrogenpolysiloxane having at least three Si--H radicals, (D) fumed silica, and (E) an addition reaction catalyst forms a seal member having improved strength and reduced compression set and useful for sealing a polymer electrolyte fuel-cell separator.