A model for anion-exchange membranes used in alkaline fuel cells was derived based on the thermodynamic of irreversible processes theory. The tests carried out in a simplified fuel cell model included combinations of operation conditions such as feed gases relative humidity, pressure in the electrodes and current density. In general, the membrane model represented well the different transport modes that can occur for wide humidity ranges especially when it is in equilibrium with water vapor, liquid water or both. It was found that the application of pressure gradients to the membrane decreases the water molar flux values through it and small current densities increase them.