This study focuses on the influence of oxygen addition on ethanol steam reforming (ESR) reaction performed in a dense Pd-Ag membrane reactor (MR) for producing hydrogen directly available for feeding a polymer electrolyte membrane fuel cell (PEMFC). in particular, oxygen addition can prevent ethylene and ethane formation caused by dehydration of ethanol as well as carbon deposition. The MR is operated at 400 degrees C, H2O:C2H5OH = 11:1 as feed molar ratio and space velocity (GHSV) similar to 2000 h(-1). A commercial Ru-based catalyst was packed into the MR and a nitrogen stream of 8.4 x 10(-2) mol/h as sweep gas was flowed into the permeate side of the reactor. Both oxidative ethanol steam reforming (OESR) and ESR performances of the Pd-Ag MR were analyzed in terms of ethanol conversion to gas, hydrogen yield, gas selectivity and CO-free hydrogen recovery by varying O-2:C2H5OH feed molar ratio and reaction pressure. Moreover, the experimental results of the OESR and ESR reactions carried out in the same Pd-Ag MR are compared in order to point out the benefits due to the oxygen addition. Experimentally, this work points out that, overcoming O-2:C2H5OH = 1.11, ethanol conversion is lowered with a consequent drops of both hydrogen yield and hydrogen recovery. Vice versa, a complete ethanol conversion is achieved at 2.5 bar and O-2:C2H5OH = 1.11, whereas the maximum CO-free hydrogen recovery (similar to 30%) is obtained at O-2:C2H5OH = 0.6:1. (c) 2009 Published by Elsevier Ltd on behalf of Professor T. Nejat Veziroglu.