The reversible transition of the orthorhombic to the cubic phase in powdered MClO(4) (where M is K, Rb, Cs) was followed by a home-made simultaneous AE-DTA apparatus. It was found that the mechanical events occurring prior to the transition as well as by the transition itself in both directions are detected by the AE method. Repeated AE runs reduced the cumulative counts for both transitions, finally giving almost constant values. The cumulative counts for the backward transition were greater than in the forward one.