Results of an electrochemical impedance spectroscopy study of nickel-iron codeposition are presented. Impedance spectra are influenced in a complicated manner by the applied current density, rotation speed, and concentration of ferrous ions. Spectra are particularly sensitive to current density and ferrous-ion concentration. As a basis for comparison, an experimental and theoretical study of nickel deposition from an iron-free bath is presented. It is found that it is the inductive loop of the impedance spectra that changes most dramatically in the presence of ferrous ions.