The influence of cobalt addition on precipitation of copper in a high-strength stainless steel was investigated using three-dimensional atom probe tomography. A decrease in copper precipitate size with narrowed distribution was observed in cobalt-containing steel. The concentration profile across the precipitate-matrix interface indicated rejection of cobalt atoms from the copper precipitates. This behavior was explained with first-principles calculations using energy-minimization criterion. © 2012 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.