Electrochemical impedance spectroscopy was performed on copper interconnect plating baths during deposition to study their degradation (ageing). A kinetic-based model was used to simulate the impedance scans by taking into account the organic additives in the reaction mechanism. Also, an equivalent circuit analysis was performed to characterize the deposition process in terms of resistive and capacitive components. Experimental results for two chemistries indicate that the low-frequency impedance relaxations change as the plating bath ages. Impedance diagrams calculated from the kinetic model resulted in a reasonable fit with the experimental impedance scans. In addition, the low-frequency capacitive and inductive impedance loop diameters are proposed as parameters to follow bath ageing. From these results, a monitoring method is proposed which could be applied to an industrial production line.