Oxygen reduction in the presence of potassium ions has been identified as a promising candidate for the cathode reaction in aprotic metal-air batteries due to its inherent reversibility. Here, we explore the kinetics of the oxygen reduction reaction in a dimethyl sulfoxide-based electrolyte using rotating ring-disk electrode measurements as well as differential electrochemical mass spectrometry. Thorough kinetic analysis reveals that the oxygen reduction in presence of K(+ )behaves like an ideal model system and that the usual relationships for reversible reactions are applicable. From the comparison of different electrode materials it has been concluded that the electrode material has an influence on the kinetics of the reduction of oxygen to superoxide, which challenges the picture of a simple outer-sphere mechanism. In fact, the reduction of oxygen to superoxide is most facile at glassy carbon electrodes, whereas it is significantly lower at gold electrodes. Rate constants are reported for gold, platinum and glassy carbon. Adding potassium perchlorate to an inert, tetrabutylammonium-containing electrolyte showed a significant stabilization of the superoxide via a shift of the half-wave potentials of the reduction to more positive potentials. This stabilization of superoxide by potassium as compared to tetrabutylammonium shows that the current interpretation of the cation's effect via Pearson's acid-base (HSAB) concept has to be used with great caution. From a detailed analysis of the effect of K+, the equilibrium constant of ion-pairing between superoxide and K+ has been determined as 725 L mol(-l). Using isotopically labelled F11 8 0 it has been shown that the formation of potassium peroxide is alleviated in the presence which is attributed to a further reaction of the peroxide with water. This in contrast to Li+-containing electrolytes, where water has the opposite effect and prevents peroxide formation. Finally, the unexpected consumption of oxygen has been observed during the oxidation of species deposited onto the surface of the electrode, which has been correlated to the amount of CO2 evolution. (C) 2019 Published by Elsevier Ltd.