The spontaneous, sonication-driven conversion of a molecular manganese vanadium oxide water oxidation catalyst, (n-Bu4N)3[Mn4V4O17(OAc)3] × 3H2O, into colloidal manganese vanadium oxide particles (average particle size ca. 70 nm) together with their stability and chemical water oxidation activity is reported. The nanoparticulate metal oxide colloid (approximate composition: VMn5O10·ca. 6H2O·ca. 0.2nBu4N+) is formed spontaneously when the molecular precursor is sonicated in water. The particles show water oxidation activity when combined with CeIV as the oxidant, are stable even under highly acidic reaction conditions and can be recovered and reused.