In this work, the performance of a High Temperature (HT) Polymer Electrolyte Fuel Cell (PEFC) stack for co-generation application was investigated. A 3 kW power unit composed of two 1.5 kW modules was designed, manufactured and tested. The module was composed of 40 composite graphite cell with an active area of 150 cm(2). Composite Membrane Electrode Assemblies (MEAs) based on Nafion/Zirconia membranes were used to explore the behavior of the stack at high temperature (120 degrees C). Tests were performed in both pure Hydrogen and H-2/CO2/CO mixture at different humidification grade, simulating the exit gas from a methane fuel processor. The fuel cells stack has generated a maximum power of 2400W at 105 A with pure hydrogen and fully hydrated gases and 1700W at 90 A by operating at low humidity grade (95/49 RH% for H-2/Air). In case the stack was fed with reformate simulated stream fully saturated, a maximum power of 2290 W at 105 A was reached: only a power loss of 5% was recorded by using reformate stream instead of pure hydrogen. The humidification grade of Nafion membrane was indicated as the main factor affecting the proton conductivity of Nafion while the addition of the inert compound like YSZ, did not affectthe electrochemical properties of the membrane but, rather has enhanced mechanical resistance at high temperature. Copyright (C) 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.