Cantilever curvature was used to examine stress generation during the electrodeposition of Co onto (111)-textured Au from 0.1 mol/L NaClO4 + 0.001 mol/L Co(ClO4)(2) (pH = 4.8) in films measuring less than 25 nm in thickness and at Co current efficiencies ranging from 65% to 90%. XRD analysis indicates that the Co is face-centered cubic and maintains the (111) texture of the Au substrate, suggesting epitaxial but not pseudomorphic growth on the Au. The stress-thickness product showed a -0.2 N/m compression in the first monolayer followed by increasing tensile stress as the film thickened. The initial compressive stress is most likely due to surface and interface stresses that dominate at submonolayer coverage. Steady state tensile stress, ranging from 0 to + 450 MPa, developed in continuous Co films and showed a strong dependence on electrode potential. However, over this potential range there is no change in grain size or growth rate, factors typically associated with tensile stress. We attribute the tensile stress and its potential dependence to the formation of hydrogen stabilized vacancies, defects found to be quite stable in fcc Fe-group metals and alloys deposited under conditions of H+ discharge. (C) The Author(s) 2016. Published by ECS. This is an open access article distributed under the terms of the Creative Commons Attribution 4.0 License (CC BY, http://creativecommons.org/licenses/by/4.0/), which permits unrestricted reuse of the work in any medium, provided the original work is properly cited. All rights reserved.