Measurements of the elastic (Young's) modulus, mechanical losses and d.c. electrical conductivity as a function of the temperature in Aurivillius type structure ferroelectric ceramics have been performed. The results suggest that the anomalies appearing in macroscopic complex elastic modulus and electrical conductivity near the temperature of 300 degrees C, do not correspond to a ferro- paraelectric or ferro-ferroelectric phase transition. These anomalies could be caused by a change in the defect structure coming from unstable Bi3+ cation. (C) 1999 Elsevier Science Limited. All rights reserved.