NOVELTY - Rubber composition (I) suitable for sealing a periphery of at least one side of a polymer electrolyte fuel-cell separator, comprising organopolysiloxane, organohydrogenpolysiloxane, fumed silica, an acid resistant inorganic filler and a catalytic amount of an addition reaction catalyst.    USE - A sealing rubber composition for use with polymer electrolyte fuel-cell separators.    ADVANTAGE - The sealing rubber composition forms a seal member featuring little or no leaching of low molecular weight organopolysiloxanes having a degree of polymerization of up to 10, especially non-functional organopolysiloxanes having a degree of polymerization of up to 10 which can be readily leached in water or a mixture of water and ethylene glycol.    DETAILED DESCRIPTION - Rubber composition (I), comprising (parts weight):    (A) organopolysiloxane having at least two alkenyl radicals each bonded to a silicon atom per molecule (100);    (B) organohydrogenpolysiloxane having at least three hydrogen atoms each bonded to a silicon atom per molecule (0.5-20);    (C) fumed silica having a specific surface area of 50 to 400 m2/g (5-40);    (D) an acid resistant inorganic filler other than fumed silica having an average particle size of up to 50 microns (10-200); and    (E) a catalytic amount of an addition reaction catalyst.    INDEPENDENT CLAIMS are also included for:    (1) a seal member for a polymer electrolyte fuel-cell separator, comprising the rubber composition (I) in the cured state;    (2) a polymer electrolyte fuel-cell separator comprising a metal plate, and a seal member formed along a periphery on at least one side of the metal plate by compression molding, casting, injection molding, transfer molding, dipping, coating or screen printing the rubber composition (I), and curing; and    (3) a polymer electrolyte fuel-cell separator comprising a substrate including an electrically conductive powder and a binder, and a seal member formed along a periphery on at least one side of the substrate by compression molding, casting, injection molding, transfer molding, dipping, coating or screen printing the rubber composition (I) and curing.