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Thermodynamic behaviour of the low­-temperature proton exchange membrane fuel cell (PEM FC) was under investigation. Different types of behaviour were observed depending on the thermal treatment of the FC. In the case of the relatively slow heating of reactant gases, the thermodynamic behaviour of the FC resembles the behaviour of the reversible FC, with an inverse proportionality between the open­-circuit voltage and temperature during the heating­-cooling cycle. In contrast, heating of the whole FC frame leads to predominantly direct proportionality during the heating­-cooling cycle and, in addition, to hysteresis.