An electrochemical impedance spectroscopy study of nickel deposition onto a rotating disk from a coumarin-containing Watts bath is presented. A model that assumes (i) adsorption of coumarin follows a Langmuir isotherm, (ii) that coumarin is consumed by incorporation into the deposit, and (iii) that metal-ion intermediates compete with the leveling agent for surface sites is presented and shown to be in reasonable accord with experiment. However, discrepancies in the spectra may provide a more reliable means of determining kinetic parameters for the description of leveling agents than steady-state polarization experiments is also discussed.