Electrohydrodynamic (EHD) impedance spectroscopy has been used to study the influence of coumarin on nickel electrodeposition. The low-frequency limit of the phase shift of the EHD-impedance experiments was found to be 180 degrees; this result is consistent with the observation that the nickel deposition rate decreases as the (mass-transfer-limited) flux of coumarin to the electrode increases. The experiments were analyzed with a model (developed previously) that is consistent with existing theories describing the influence of leveling agents. The model successfully predicts variations in spectra with disk rotation speed and current density but is less successful at predicting variations with the bulk coumarin concentration. The use of the EHD-impedance method for the determination of interfacial-kinetic and transport parameters relevant to models of leveling agents is demonstrated.