Single-crystal (Mn,Co)(3)O-4 octahedra were synthesized through a novel precipitation-aging method. Well-defined single-crystal octahedra can be formed by carefully controlling the precipitation-dissolution equilibrium, oxygen concentration, and solution temperature during synthesis to match the rates of spinel nucleation and octahedra growth. The single-crystal (Mn,Co)(3)O-4 octahedra expose {111} and {011} facets of cubic and tetragonal phases, respectively, depending on the Mn/Co ratio. However, only single-crystal octahedra of Mn2CoO4 and Mn2.5Co0.5O4 with exposed {011} facets show the highest selectivity towards 4e(-) and 2e(-) oxygen reduction reactions (ORR) in alkaline solution, respectively. Furthermore, the single-crystal Mn2CoO4 octahedra with exposed {011} facets shows similar to 30 times higher area specific activity for ORR than that of nanoparticles with random/mixed facets. The high electrocatalytic activity and selectivity towards ORR are correlated with the octahedral shape, the exposed facet, and the ratio of cations on the exposed facets (especially the octahedrally coordinated Mn-4 cations). This facet dependent catalytic performance provides a new route for obtaining highly selective and active ORR electrocatalysts. (C) 2014 Elsevier Ltd. All rights reserved.