We report on the sub-nanometer scale characterization of Co-12Ti-4Mo and Co-12Ti-4Cr (at.%) model alloys. Atom probe tomography reveals that Co and Cr partition to gamma, whereas Ti and Mo to gamma'. Additions of Mo and Cr to the reference Co-12Ti system lead to strong increases in gamma' volume fraction by about 25% and 12%, respectively. Element-specific spatial distribution maps along the  direction of the L1(2)-ordered gamma' phase reveal that both Mo and Cr preferentially replace Ti on its sublattice. The remaining excess Ti is available for formation of additional gamma', resulting in enhanced gamma' volume fractions. (C) 2018 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.