Monometallic Pt catalysts (17% wt) were prepared over Vulcan carbon (VC) and multiwall carbon nanotubes (NT). These supports were functionalized with citric or nitric acid. The characteristics of the different functionalized and non-functionalized supports were analyzed to determine the influence of the functional groups over the electrocatalytic behavior of the Pt monometallic catalyst. The functionalization treatment with nitric acid increases the concentration of strong and weak functional groups, whereas citric acid promotes the formation of acid groups with intermediate acidic strength. Catalysts prepared by conventional impregnation (CI) and supported on functionalized VC and NT display much higher electrochemical active areas than those supported on the corresponding non-functionalized carbons. Both the decrease of the metallic particle sizes and the changes in the metal-support interaction due to the presence of the functional groups would lead to a better electrochemical behavior. (C) 2016 The Electrochemical Society. All rights reserved.