Catalase was investigated as a possible catalyst of the electrochemical reduction of oxygen on glassy carbon electrodes. Four different electrode / enzyme interfaces were tested: free enzyme in solution, enzyme immobilised on the electrode surface with bovine serum albumin and glutaraldehyde, or with Nafion(R), and enzyme adsorbed from solution in dimethylsulfoxide. The experimental data were analysed with a theoretical model. The model was based on two successive two-electron electrochemical reactions: reduction of oxygen to hydrogen peroxide and reduction of hydrogen peroxide to water, which were coupled to the diffusion of oxygen and hydrogen peroxide, and to the catalase-catalysed disproportionation of peroxide in solution. In most cases, the presence of catalase in solution or on the electrode surface provoked only a moderate current increase. The model demonstrated that disproportionation of hydrogen peroxide in solution fully explained this current increase. On the contrary, when catalase was adsorbed from dimethylsulfoxide on the surface of electrodes that did not undergo any electrochemical pre-treatment, a high current increase was obtained. In this case, catalase efficiently catalysed oxygen reduction via direct electron transfer with the electrode. The effects of the electrochemical pre-treatment on oxygen reduction and on the occurrence of catalase-catalysed direct electron transfer were discussed. (C) 2000 Elsevier Science B.V. All rights reserved.