A liquid-gas separator assembly is used in separating gas bubbles from a liquid coolant which liquid coolant is used in a polymer electrolyte membrane (PEM) fuel cell power plant. The assembly includes a cylindrical housing containing a central tube which is surrounded by an annular chamber. The annular chamber is defined by the outer surface of the central tube and the inner surface of the cylindrical housing. An inlet line injects a stream of the coolant from the fuel cell stack area of the power plant into the bottom of the central tube in a tangential flow pattern so that the coolant and gas bubble mixture swirls upwardly through the central tube. The swirling flow pattern of the coolant and gas bubble mixture causes the gas bubbles to separate from the liquid coolant so that the gas in the mixture will migrate to the central portion of the swirl tube and the liquid component of the mixture will centrifugally migrate to the inner wall of the swirl tube. The gaseous component of the separated mixture is then expelled from the housing through an outlet in the upper portion of the housing, and the coolant liquid descends through the annular chamber to the bottom of the housing where the coolant liquid will be returned to the cell stack area of the power plant. The gas bubbles may include reactant from either the cathode or the anode. The separator of this invention is particularly useful in mobile applications since it is less sensitive to vehicular acceration forces.