A liquid electrolyte fuel cell system (10) comprises at least one fuel cell with a liquid electrolyte chamber between opposed electrodes, the electrodes being an anode and a cathode, and means (30, 32) for supplying a gas stream to a gas chamber adjacent to the cathode and withdrawing a spent gas stream (38) from the gas chamber adjacent to the cathode, the system also comprising a liquid electrolyte storage tank (40), and means (42, 44, 47, 48) to circulate liquid electrolyte between the liquid electrolyte storage tank (40) and the fuel cells. In addition the system comprises a water storage tank (60) adjacent to the storage tank (40), and means (50, 51) for condensing water vapor from the spent gas stream (38), and for feeding (56) the condensed water vapor into the water storage tank (60). The water storage tank (60) has an overflow outlet (64); and a communication duct (68) linking the liquid electrolyte storage tank (40) and the water storage tank (60) below the level of the overflow outlet (60). This automatically replaces any water that evaporates from the electrolyte, without requiring any electronics.