This study investigates the relative electrochemical activity of ordered and disordered Pt-Co alloy phases coexisting in multi-phase catalyst materials. Of particular interest is the effect of the relative distribution between ordered and disordered Pt-Co alloy phases on the observed electrocatalytic activity for the oxygen reduction reaction (ORR). Three Pt-Co catalysts with identical overall composition, Pt50Co50, but with distinct distributions between two disordered face centered cubic (fcc) Pt-Co alloy phases and one ordered face centered tetragonal (fct) alloy phase are considered. Comparing the structure of the catalysts with their electrocatalytic activity for ORR suggests that the Co-rich (60-80 at% Co) disordered phase is linked to the observed 3 x activity enhancement compared to a pure Pt catalyst. If the ordered fct phase outweighs the Co-rich disordered phase the activity drops drastically. It is concluded that Co-rich disordered phases are the preferred Pt-Co alloy phases with respect to catalyst activity. (C) 2007 Elsevier B.V. All rights reserved.