Plasma electrolytic oxidation (PEO) was studied using electrochemical impedance spectroscopy (EIS) and transient waveform analysis. First, EIS measurements on coatings prepared under constant current density were conducted in the working electrolyte. The EIS analysis showed that the coating structure changed with the termination voltage. Second, the pulse waveform of the power supply was recorded during the PEO process. It showed that a charging process occurred across the breakdown coating at the beginning of the pulse. The present study analysed this charging process in detail.