Over the years, studies have analyzed the composition of the catalyst layer using commercial platinum catalyst, supported on Vulcan XC72 with 20% of metal loading (Pt/C 20%(Mw)), and found that values between 20 and 40% of Nafion ionomer related to the mass of the catalyst layer (% NIw) have resulted in more efficient electrodes for PEMFC. Recent studies with catalysts synthesized on Vulcan XC72 resulted in 59% Nlw as the best formulation. In this work, the commercial and the synthesized Pt/C 20%m, catalyst were evaluated by Gas Pycnometry, Gas Adsorption (through BET and BJH), and Mercury Intrusion Porosimetry. The results showed volumetric differences between the Vulcan XC72 used in commercial catalyst and the Vulcan XC72 commercially available for synthesis (as purchased). These differences impair the synthesized catalyst in comparison with the commercial one. Therefore, the relationship between the quantities of catalysts and Nafion ionomer on the catalyst layers must be calculated as a function of the catalysts volumes. Copyright (C) 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.