PtCo alloys have long been known to possess higher activity in the electrochemical oxygen reduction reaction compared to pure Pt. This work addresses the influence of the chemical order on the electrocatalytic activity of PtCo alloy, the phenomenon, which has been already documented in the literature but yet poorly understood. Single crystalline PtCo(0 0 1) alloy films were epitaxially grown on Mg(0 0 1) substrates and used as model electrodes to study the influence of the chemical order on the electrochemical oxygen reduction reaction. Electrodes with close Pt:Co atomic ratios in the bulk and near surface region show grossly different electrocatalytic activities against the chemical order, the ordered L1_{0} phase being ca. 7 times more active than the disordered fcc A1 structure. Electronic structure calculations and quantum mechanical theory of electron transfer are utilized to provide a rational for this remarkable phenomenon.