The aim of this work is to obtain deeper insight into the mechanism of the protective action of three imidazole-based corrosion inhibitors. Investigations were performed on copper in 3% NaCl, by electrochemical impedance spectroscopy and electrochemical quartz crystal microbalance measurements. The kinetic changes in the corrosion processes were monitored over time. In spite of similar molecular structure, differences in the inhibiting mechanism of three imidazole compounds were observed. The two inhibitors with a tolyl substituent decreased the copper corrosion rate due to the formation of a thin adsorbate layer; however, slow formation of a thick layer was observed for 4-methyl-1-phenyl imidazole. From electrochemical impedance spectroscopy studies, it was observed that all three inhibitors protected the copper efficiently, and in some cases, the degree of the protection increased with time.