We have measured the stress response as a function of overpotential in films measuring less than 25 nm in thickness, from 0.1 mol/L NaClO4 + 0.001 mol/L Co(ClO4)2 (pH = 4.8) and at Co current efficiencies ranging from 65% to 90%. XRD analysis indicates that under these deposition conditions the Co is face-centered cubic and maintains the (111) texture of the Au substrate, suggesting epitaxial but not pseudomorphic growth on the Au. Figure 1(a) shows the measured stress-thickness product over the deposition/stripping cycle for several deposition potentials. The corresponding steady-state stress associated with these curves varies from 0 to 450 MPa tensile, with much of the increase occurring in the potential range of -1.22 V to -1.24 V vs. SSE. However, over this potential range there is no change in grain size or growth rate, factors typically associated with tensile stress generation. Interestingly, the corresponding stripping voltammetry (Figure 1(b)) shows than an additional stripping wave appears in films deposited at potentials that correspond to the large increase in tensile stress. These additional stripping waves are typically associated with hydrogen in the film, suggesting that tensile stress can be attributed to the co-generation of hydrogen. Interstitial hydrogen is typically associated with compressive stress, or post-deposition tensile stress as the hydrogen is liberated from the film. This talk will examine additional sources of hydrogen-induced tensile stress, such as the presence of hydrogen stabilized vacancies, defects found to be quite stable in fcc Fe-group metals and alloys deposited under conditions of H+ discharge. As a counter argument, we will consider the possibility that co-generation of hydrogen may alter the energetics associated with nuclei coalescence, through modification of the Co surface energy or subsequent grain boundary energy, rather than through changes in grain size.