Mullite-type alkali aluminates (KyNa1–y)0.67Al6O9.33 were synthesized from amorphous Al and alkali nitrates by sol-gel techniques. Rietveld refinements of six members of the solid solution series (y = 0.0, 0.2, ..., 1.0), together with Fourier syntheses and grid search analyses show that the Na and K atoms reside in the vacant Oc sites, with K at 1/2, 0, 1/2 and Na on a split site off the special position. The number of alkali atoms is restricted to 2/3 atoms per unit cell due to crystal chemical constraints. Consequently, unlike the aluminosilicate mullites, alkali mullites do not form a solid solution series with varying oxygen composition. All compounds studied here crystallize in space group Pbam with lattice constants ranging from a = 7.6819(4) Å, b = 7.6810(4) Å, c = 2.91842(8) Å for the Na aluminate to a = 7.6934(3) Å, b = 7.6727(3) Å, c = 2.93231(7) Å for the K aluminate mullite.