A reinvestigation of the system K2O–UO3 by diffraction methods and i.r. spectroscopy shows that there are two well defined polyuranate phases. Both are based structurally on layers of uranium in hexagonal array, as found in U3O8 and K2U2O7. One polyuranate, isolated in crystalline form is of the diuranate type, with anionic layers 7.045 Å apart, the spacing determined by cation size and packing dimensions of the uranyl group. It has a composition near K2O ∙ 3 UO3, but probably a more complex formula. The second, not isolated pure, has K2O : UO3 < 1:8, with the layers continuously linked through shared uranyl oxygen atoms, as in α-UO3. The structural principles of these pseudo-hexagonal layer structures are discussed.