The first discrete mixed platinum(IV)-gold(III) oxoanion [PtIV2AuIII3O6((CH3)2AsO2)6]- (1) was synthesized by reaction of H2Pt(OH)6 with H[AuCl4] in a simple one-pot procedure in aqueous solution at pH 7 and comprises two equivalent PtIVO6(As(CH3)2)3 units which are linked by three square-planar AuIIIO4 units. Polyanion 1 could be isolated as a potassium or sodium salt in good yield, which were structurally characterized in the solid state by single-crystal XRD and TGA, and in solution by multinuclear (1H, 13C, 195Pt) NMR, indicating that polyanion 1 is stable in solution, which was confirmed by ESI-MS studies. The sodium salt of 1 undergoes a clean single-crystal-to-single-crystal (SCSC) structural transformation upon rehydration and dehydration.