An electrochemical impedance study of the electrodeposition of nickel in the presence of coumarin is presented. A transient model is developed for the inhibition of nickel deposition by a leveling agent and is compared to experimental data. The model is successful at predicting the dependence of impedance data on inhibitor concentration, rate of electrolyte agitation, and applied potential. It is found that the diameter of the inductive loop of the impedance spectra is maximum for conditions corresponding to maximum leveling.