In the presented work we investigate the influence of de-alloying carbon supported PtxCo1-x alloy nanoparticles (NPs) on their oxygen reduction reaction (ORR) activity. In our study we used a modification of a recently developed colloidal synthesis approach in order to prepare NPs of similar size, i.e. around 2 nm in diameter, but varying initial Pt:Co ratios. The results show that using a surfactant is favorable to control the initial Pt:Co ratio. The initial composition of the catalyst however is lost upon introduction into an acid environment, i.e. the well-known effect of chemical de-alloying occurs. Interestingly, despite the fact that due to de-alloying almost all Co is removed from the PtxCo1-x NPs, a clear trend of increasing ORR activity with increasing initial Co content is observed. The results indicate that the process of chemical de-alloying is favorable for producing active ORB. catalysts. Copyright (C) 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.