We report a synthetic electrochemical strategy to deliberately modify the catalytic reactivity of Pt bimetallic surfaces. The strategy consists of voltammetric surface dealloying of the non-noble constituent from Pt-poor bimetallic precursor compounds. We exemplify this method by dealloying carbon-supported Pt25Cu75 alloy nanoparticle precursors and testing the resulting active catalyst phase for the oxygen reduction reaction (ORR). We show that dealloyed Pt-Cu electrocatalysts exhibit an extraordinary increase in intrinsic reactivity of 4-6 times as compared to pure Pt electrocatalysts. Our results indicate that electrochemical treatment of the alloy precursors selectively dissolves Cu near the particle surface. The partially dealloyed particles constitute the active catalyst phase. While Cu is retained in the core of the particles after dealloying, the essentially pure Pt surface suggests a core-shell structure of the active catalyst. Geometric effects, such as exposure of more active crystallographic facets or a more favorable Pt-Pt surface interatomic distance are proposed to play a key role in the enhancement mechanism. This work suggests that the selective electrochemical dissolution (dealloying) of non-noble components from noble metal bimetallics can serve as a general strategy toward tuning surface electrocatalytic properties.