The aim of this work is the performance study of a polimeric electrolyte membrane stack. Extreme conditions of temperature and relative humidity, such as those that may be found in practical situations (i.e., automotive), have been considered. The research has been developed by Department of Energetics of Politecnico di Milano in collaboration with Nuvera Fuel Cells Europe, under confidential agreement. In order to select the proper electrolyte that can be used to build a stack suitable for automotive applications, three different types of material have been tested in single fuel cells, under different conditions of temperature and relative humidity by mean of a climatic chamber. Both traditional and new materials have been tested in single cells of of active area. The three electrolyte materials have been tested also by measuring the protonic conductance, in different conditions of relative humidity. After these tests, an electrolyte has been chosen that was made with a coated catalyst membrane having a thickness of , which has been used to build a six-cell stack with an active area of . The performances of the stack have been evaluated, in continuous operation, with air temperatures ranging from 50°C to . A series of start-up tests has been carried out with an air temperature ranging between 0°C and .