An insoluble salt of the water oxidation catalyst [Co9(H2O)6(OH)3(HPO4)2(PW9O34)3](16-) (Co9) 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, Co9 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.