Apparatus for regulating the quantity of water present in a reaction chamber of a fuel cell system, which has a circulating aqueous solution of electrolyte and air operated cathodes. The reaction chamber includes an anode and a cathode. The regulating apparatus includes two portions, the first of which acts to control the temperature of the electrolyte solution and the second of which controls the air throughput in the system in dependence upon the battery current. The temperature control device raises or lowers the operating temperature of the electrolyte solution in response to an increase or decrease, respectively, in the concentration of the solution with respect to the predetermined value range. The output of the air pump which supplies air to the cathodes is regulated in dependence upon the current flowing between the anode and cathode.
