Due to their high activity for the oxygen reduction reaction (ORR), Pt based alloys are of considerable interest as catalysts for polymer electrolyte fuel cells (PEMFC). We present two synthesis protocols that give PtxCo1-x alloy nanoparticles (NPs) supported on a commercial carbon black support (Ketjen black EC300J) with controlled particle size, Co content and particle distribution for oxygen reduction reaction (ORR). The as prepared catalysts samples can be pre-leached in acid solution to obtain skeleton NPs and heat-treated in a reducing atmosphere to promote alloying as well as to reduce the number of low coordinated surface sites of the NPs. Without heat treatment, the catalysts exhibit an activity increase for the oxygen reduction reaction mainly due to a strain effect, whereas an improved performance due to a combined strain and ligand effect requires thermal treatment of the catalyst. (C) 2013 Elsevier B.V. All rights reserved.