Potassium beta-alumina was synthesised from potassiumm-alumina by a new technique. Thermal decomposition of a complex, trioxalatoaluminate Kx(NH4)3−x [Al(C2O4)3] ·3H2O (0.091⩽x⩽ 0.333), resulted in the formation of a fine powder of Km-alumina with mullite-like structure. The decomposition process was studied by thermal analysis and X-ray diffraction, and the phase relation of the K2O-Al2O3 system is discussed in the mid-temperature region of 600–1200°C. The a.c. conductivity, the apparent density and the lattice parameters were measured. The complex decomposition was shown to be a suitable method for Kβ-alumina synthesis.