Optical emission spectroscopy and fast video imaging of the microarc oxidation process of aluminium alloys are carried out. Optical measurements allow the establishment of relationships between the sparks aspect and the aluminium emission line intensity. The transition from spark to microarc is quantified through the decay of the aluminium line intensity. The characteristic time of that decay is shown to be strongly dependent on the current density. Scanning electron microscopy characterization shows that the oxide layer structure and growth rate are strongly dependent on the process duration and on the applied current density. Finally, the correlations that are drawn between the material analyses and the plasma measurements allow proposing some ways to enable the process control through optical measurements.