The oxygen reduction mechanism was investigated in carbonate melts of variable composition ranging from the eutectics Li-K, Li-Na to ternary Li-Na-K melts. The effect of pressure on electrode kinetics was also examined. Cyclic voltammetry, convolution voltammetry and electrochemical impedance spectroscopy have shown that oxygen reduction is mass-transfer limited in all melt compositions investigated. It was also found that pressurizing increased the concentration of oxygen electroactive species. To gain information about the reaction mechanism, the impedance spectra have been fitted to four possible equivalents circuits: (i) Randles circuit, (ii) Randles circuit extended by a parallel RC circuit due to film formation, (iii) Gerisher (homogeneous reaction) impedance, and (iv) Modified Randles circuit. The discussion of the results is based on careful analysis of fit quality and meaningfulness of fit parameters.