A carbon paste electrode (CPE) modified with sodium montmorillonite (SWy-2)-methyl viologen (MV) is developed. The intermolecular complex of SWy-2 and MV has an electrocatalytic effect on oxygen reduction. The cyclic voltammograms of the electroreduction of molecular oxygen show an enhanced current peak in an air-saturated phosphate buffer solution. In this study, the experimental parameters are optimized and the mechanism of the catalytic process is discussed. This chemically modified electrode will offer a possibility of developing a sensitive method for the determination of dissolved oxygen.