An ultrafine grained two phase Co-Cu alloy was processed by electrodeposition. The two phase Co-Cu alloy was composed of hexagonal close-packed Co-rich grains and face-centered cubic Cu-rich grains. The two phase Co-Cu alloy exhibited a high strain rate sensitivity of 0.137 and low activation volume of 3.3 b(Co)(3) or 3.1 b(Cu)(3) at room temperature. It is suggested, from the results of an atomic simulation of the Co/Cu grain boundary, that the unique deformation characteristics can be attributed to Co/Cu heterogeneous grain boundaries. (C) 2010 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.