Polyurethane coated OPC mortar specimens exposed to 1M NaCl solution were studied by impedance spectroscopy and SEM over a period of 4 years. Results showed a rapid reduction of the coating resistance over the first half year, followed by a slow decrease thereafter. The coating deteriorated starting from the surface in contact with the NaCl solution and the deterioration front reached the coating/substrate interface after around three years. Different stages of deterioration were clearly identified by impedance spectroscopy. The study demonstrates that impedance spectroscopy is a versatile and informative non-destructive method to assess and monitor the performance of surface treatments on concrete.