The present study provides a detailed discussion of mechanisms underlying anomalous gas evolution and changes in electrolyte pH during Plasma Electrolytic Oxidation (PEO) of Al in alkaline solutions. This is important for process optimisation and control in deposition of wear and corrosion resistant PEO coatings, which currently attract significant interest. The oxidation was performed under galvanostatic conditions in the range of current densities from 400 to 1500 A m(-2), with concentration of aqueous alkaline electrolyte varied from 0.5 to 2 g l(-1) KOH. Electrolyte pH was evaluated before and after each PEO treatment. The volume of anodic gas evolved during the process was also measured, with the composition of the gaseous products being subsequently analysed using a quadrupole mass spectrometer. It was proposed that, within the studied ranges of experimental parameters, the evolution excessive oxygen is primarily caused by peroxide decomposition due to the interaction with both hydroxyl ions and radicals at the discharge-electrolyte interface. The interaction with the ions also results in electrolyte acidification, although this process is likely to be contributed by water decomposition due to the ion bombardment from discharge. (C) 2007 Elsevier B.V. All rights reserved.