β-Al2O3 formation fromm-Al2O3 was found by a new convenient technique. By thermal decomposition of a complex compound, trioxalatoaluminate Na x (NH4)3−x [Al(C2O4)3]yH2O(0.091 ⩽x ⩽ 0.333;y= 3), a very fine powder ofm-Al2O3 was formed. The decomposition process was examined by thermal analysis and X-ray diffraction and the phase relation of the system Na2O-Al2O3 in the midtemperature region between 600 and 1200° C is discussed briefly. The tablet ofm-Al2O3 was fired at 1600° C for 30 min to prepare denseβ-Al2O3 ceramics, the apparent density of which was greater than 95% of the theoretical value. The sintered tablet was examined from the structural point of view and the electrical conductivity was measured by an a.c. method to verify that the procedure of the complex decomposition is a suitable technique forβ-Al2O3 synthesis.