DTA-, DSC-, and DTG-diagrams of aluminiumhydroxide chloride gels show numerous sharp and often rather regular oscillations, caused by endothermal decomposition and exothermal crystallisation subprocesses in the temperature range between 550 and 600 degrees C. In this range the formation of alpha-Al2O3, corundum, is observed. It is assumed, that the oscillations are produced by the periodic formation of dense layers of an intermediate phase, which act as diffusion barriers for the decomposition gas, and the crystallisation of these layers into corundum, connected with a reduction of transportation hindrance of the gas molecules. These processes are considered in their feedback with the decomposition reaction, by which the intermediate phase is formed.