NOVELTY - The solid polymer fuel cell system comprises humidifying apparatus comprising membrane exchange humidifier (2) containing supply stream (SS) chamber (11) and exhaust stream (ES) chamber (12), intervened with water permeable membrane (13). Membrane is capable of transferring water from ES to SS, and comprises microporous polymer and hydrophilic additive. Membrane is permeable to SS and/or ES in dry state.    USE - As solid polymer fuel cell. The membrane exchange humidifier is used for medical equipment, air conditioner.    ADVANTAGE - The membrane has favorable water transmission properties and resists transmission of reactant components. The membrane is suitable for use even in dry or wet conditions and when wet and dry gaseous are of different composition. The membrane is inexpensive, tough, dimensionally stable and bondable using adhesives or via melt bonding. The improved mechanical and welding properties of the membrane allow for a simpler humidifier configuration.    DETAILED DESCRIPTION - The fuel cell system comprises a solid polymer fuel cell (1) and apparatus for humidifying the reactant gas supply stream. The fuel cell has a reactant gas inlet port (5) and a reactant gas exhaust port (6). The supply stream chamber has an inlet (3a) which is fluidly connected to the reactant gas supply, and outlet (3b) which is fluidly connected to the reactant gas inlet port of fuel cell. The exhaust stream chamber has an inlet (3c) and an outlet (3d), with inlet fluidly connected to fuel cell reactant gas exhaust port. The membrane in wet state, is impermeable to supply stream and/or exhaust stream. The membrane is permeable to up to about 1% of reactant gas volume during steady state fuel cell operation. The reactant gas supply stream and exhaust stream comprises oxidant supply stream and oxidant exhaust stream, respectively. An INDEPENDENT CLAIM is also included for method for humidifying two fluid streams using a membrane exchange apparatus.    DESCRIPTION OF DRAWING(S) - The figure shows schematic diagram of solid polymer fuel cell system.    Solid polymer fuel cell (1)    Membrane exchange humidifier (2)    Inlet of supply stream chamber (3a)    Outlet of supply stream chamber (3b)    Exhaust stream chamber inlet (3c)    Exhaust stream chamber outlet (3d)    Fuel cell reactant gas inlet port (5)    Fuel cell reactant gas exhaust port (6)    Supply stream chamber (11)    Exhaust stream chamber (12)    Water permeable membrane (13)