The plasma electrolytic oxidation (PEO) of aluminium alloys is investigated for different electrical working conditions using a pulsed bipolar current supply. A particular attention is paid to the effect of the anodic current density (from 10 to 90 A dm(-2)) and current pulse frequency (from 100 to 900 Hz) on the resulting oxide layer. Micro-discharges are characterized during the process by means of fast video imaging with a time and a space resolution of 8 mu s and 0.017 mm(2), respectively. Correlations are established between the micro-discharge characteristics (surface density, lifetime and size) and the elaborated oxide layers (morphology, growth rate and surface roughness). The highest coating growth rate measured (2.1 mu m min(-1)) is achieved with the combination of the highest current density (75.7 A dm(-2)) and the highest current pulse frequency (900 Hz). Within these specific current conditions it is concluded that the detrimental effects of numerous micro-discharges are minimized. The results also show that the surface roughness may be largely affected by the presence of long-lived and large micro-discharges which develop over the processed surface. The strongest micro-discharges (live duration up to 0.3 ms and cross-sectional area up to 1 mm(2)) are mainly observed with the combination of the highest current density (75.7 A dm(-2)) and the lowest current pulse frequency (100 Hz).