Use of crosslinkable silicone rubbers, stable against degradation as sealants in fuel cells

Use is claimed as a sealant for fuel cells or fuel cell stacks of an elastomeric crosslinkable composition comprising: (A) a polyorganosiloxane (I) with two or more alkenyl groups and a catalyst; and (B) a polyorganosiloxane (II) with two or more silicon-bonded hydrogen atoms and an organic or organosilicon sulfur-compound additive.