Plasma electrolytic oxidation (PEO) of magnesium alloy is performed in NH4F and alkaline-silicate electrolytes within various waveforms of bipolar current pulses to investigate the role of the cathodic bias on both the growth mechanism of PEO layers and the behavior of micro-discharges. A particular attention is paid to cathodic micro-discharges and their ignition (or not) according to process conditions. Although cathodic micro-discharges have a detrimental effect on PEO layers, cathodic bias is necessary to improve the growth of oxide layers. Conditions that promote cathodic breakdown are discussed and a mechanism of layer cathodic breakdown is proposed. Cathodic micro-discharges, and to a greater extent the applied cathodic bias, influence the anodic breakdown coming immediately after, and therefore, the process behaviour during the anodic bias period.