Pt-Co alloy supported on carbon (Pt-Co) was electroactivated with or without SiW12O404-, and used as cathodes for the hydrogen evolution reaction (HER) in 3 M KOH at 70 degreesC. These electrodes were characterised through neutron activation (NA) and electrochemical impedance spectroscopy (EIS). The best HER performances were observed on Pt-Co electroactivated with STA. The enhanced HER electrocatalytic activity observed on Pt-Co electroactivated with STA was attributed to an increase in its active surface area and to its surface chemical composition. (C) 2001 International Association for Hydrogen Energy. Published by Elsevier Science Ltd. All rights reserved.