The Nafion(R) 117 membrane was investigated by measuring the emf as a function of the temperature difference between the electrodes in the following cell : Ag(s,T-1,)AgCl(s,T-1)HCl(aq,c,T-1)Nafion(R)117HCl(aq,c,T-2)AgCl(s,T-2)Ag(s,T-2) The reversible entropy transferred due to the membrane, was S*(HM)-t(w)S(w) = 13.4 +/- 0.2 J K-1 mol(-1) at 25.0 degrees C, where S*(HM) is the transported entropy of protons in the membrane and S-w is the partial molar entropy of water in the bulk solution. The membrane transference coefficient of water, measured by the streaming potential technique, was t(w) = 2.6 +/- 0.2. The results were used to calculate the reversible heat effects at the electrodes in the solid-polymer-electrolyte fuel cell (SPEFC). We concluded that water management in the fuel cell had a strong impact on the heat effects. The total operation of the SPEFC was discussed in view of the new results. The stack method presented here, is simple and accurate for finding the Seebeck coefficient in electrochemical cells with ion-exchange membranes.