Aiming at a better understanding of the air electrode processes in Mg-air batteries, the influence of Mg2+ ions is studied on the oxygen reduction (ORR) and oxygen evolution (OER) reactions in the ionic liquid 1-butyl-1-methylpyrrolidinium bis(trifluoromethanesulfonyl) imide (BMP-TFSI) by differential electrochemical mass spectrometry (DEMS). The numbers of electrons transferred per reduced O-2 molecule are determined in cyclic voltammetry (CV) and potential step measurements, both in absence and presence of Mg2+. The numbers of transferred electrons indicate that in neat BMP-TFSI a transition from superoxide to peroxide formation takes place when applying more negative potentials. In the presence of Mg2+, the formation of superoxide can be found in potential step measurements to potentials >= -0.4 V (vs. Mg/MgO) and in CV measurements on largely passivated electrode surfaces. In contrast, mainly peroxide is formed when stepping to -1.2 V (vs. Mg/MgO) and at the beginning of the ORR in CV measurements. The formation of superoxide in the presence of Mg2+, on the passivated electrode, is found to be partly reversible, as indicated by the evolution of O-2 detected via DEMS. (C) 2018 Elsevier Ltd. All rights reserved.