A series of Cobalt-polyoxometalates (Co-POM) have been incorporated into carbon paste electrodes as insoluble salts of Cs+ or Ba2+. Specifically, we included the lacunary Keggin-type derivatives [Co(H2O)(PW11O39)]5– (CoPW11), [Co(H2O)(SiW11O39)]6– (CoSiW11), [Co(H2O)(CoW11O39)]7– (CoCoW11), [Co4(H2O)2(PW9O34)2]10– (Co4P2W18), [Co4(H2O)2(SiW9O34)2]12– (Co4Si2W18), and [Co9(H2O)6(OH)3(HPO4)2(PW9O34)3]16– (Co9); and the lacunary Dawson-Wells derivative [Co4(H2O)2(P2W15O56)2]16– (Co4P4W30). The activity and stability of these composite electrodes towards water oxidation in acidic media (pH < 1) was examined by electrochemical methods and post-catalytic characterization. Our results indicate the stability of these catalysts in these conditions depends on composition, since only those species containing PV as heteroatom appeared to be stable. All SiIV and CoIII derivates showed rapid decomposition, regardless their nuclearity or molecular structure. All stable derivatives exhibited consistent activity, favored by high nuclearity and overall total charge, with Keggin derivatives being intrinsically more active than the corresponding Dawson-Wells analogues.