The title compound, K4[MoVIO2(SO4)3], was precipitated from a melt of molybdenum(VI) oxide and potassium sulfate in potassium disulfate. The compound contains monomeric [MoVIO2(SO4)3]4− anions, with the MoVI atom, both oxide ligands, and the S atom and both ligating O atoms of the bidentate sulfate group lying on a crystallographic mirror plane. One of the potassium cations is nine-coordinate, while the other is eight-coordinate