NOVELTY - Fuel cell system comprises two humidifiers, one associated with fuel source and the other associated with oxidant source. At least one of the humidifiers has a stack of chambers or flattened tubes with the chambers or tubes in the stack being spaced from one another. Fins in the stack extend between the chambers or flattened tubes in the spaces.    USE - Producing electric power for electric motors employed to provide propulsion for a vehicle.    ADVANTAGE - Assures that membranes will not be dried out by the incoming gaseous reactants and the reactant gases will be delivered to the fuel cell at a desired elevated (relative to ambient) target dew point temperature range, typically 63 degreesC. It also provides assurance that the membranes are not subjected to undesirable drying out over wide range of operating conditions and at the elevated temperatures at which fuel cells typically operate. It maintains a desired, high degree of efficiency of operation.    DETAILED DESCRIPTION - A fuel cell system comprises:    (a) a fuel cell having electrodes, membranes and heat exchange medium flow path;    (b) a source of gaseous fuel for one of the electrodes;    (c) a source of gaseous oxidant for the fuel and for another of the electrodes; and    (d) two humidifiers, one associated with the fuel source and the other associated with the oxidant source.    At least one of the humidifiers has a stack of chambers or flattened tubes with the chambers or tubes in the stack being spaced from one another. Fins in the stack extend between the chambers or flattened tubes in the spaces. A housing containing the stack and having a first inlet and first outlet to and from the chambers or tubes is connected in the heat exchange medium flow path. A second inlet and a second outlet from the housing are connected between one of the sources and the corresponding one of the electrodes. A hydrophilic surface is provided in the fins. A spray generating or dispersing device is provided for aqueous material in the second inlet.    An INDEPENDENT CLAIM is also included for a humidifier for use in a fuel cell comprising a core in the form of a rectangular solid including spaced heat exchange medium flow paths alternating with gas and water flow paths. Each of the heat exchange medium flow path includes a pair of spaced rectangular sheets, aligned dimples on the sheets engaging and bonded to each other, a rectangular flange structure sealing the edges of the sheets in each pair, and gaps in two opposite ends of the flange structure defining an inlet and an outlet to the space between the sheets of each pair. Each of gas and water flow paths is defined by a space between two adjacent ones of the sheets in adjacent pairs, a rectangular fin in the space, and two parallel bars of flanges, one at each of two opposite sides of the fin and extending the length, and hydrophilic coating on both sides of the fin. First and second header plates are provided, one at each end of the core and the ends of the bars or flange. Two header and tank constructions are also provided, one at each of the two opposite ends and opening at the gaps and sealed to the bars of flanges and the flange structure about the gaps. A water injector is placed near an upstream one of the header plates for injecting water into the gas and water flow paths.    DESCRIPTION OF DRAWING(S) - The figure shows a type of fuel cell system in which a humidifier may be employed.    Fan (20)    Pump (26)    Temperature controlled valve (28)    Water separator (34)    Compressor (56)