The reaction of chromium ammine complexes with chromium-substituted Keggin anions in aqueous solution forms insoluble salts which were then heated to temperatures of 240 and 350 degrees C. The thermal behaviour of the ammine complexes is dominated by a strong exothermic reaction which is interpreted as a linking reaction (condensation) between chromium from the complex ammine cation and the heteropolyanion. Thermal treatment of the title compounds yields products that are soluble in water and could be crystallized as potassium salts in several cases. A chromium-caped Keggin anion is the proposed structure.