An insoluble salt of the water oxidation catalyst [Co-9(H2O)(6)(OH)(3)(HPO4)(2)(PW9O34)(3)](16-) (Co-9) has been used to modify amorphous carbon paste electrodes The catalytic activity of this polyoxometalate is maintained in the solid state. Good catalytic rates are reached at reasonable overpotentials. As a heterogeneous catalyst, Co-9 shows a remarkable long-term stability in turnover conditions. The oxygen evolution rate remains constant for hours without the appearance of any sign of fatigue or decomposition in a large pH range, including acidic conditions, where metal oxides are unstable.