Ac conductivity measurements were carried out on polycrystalline samples of a ferrimagnetic spinel (Zn0.44Mn0.56Fe2O4) and a ferroelectric perovskite (Sr0.25Bi4Ti3.25O12.75), in the temperature range 20–160 and 20–660 °C, respectively, and in the frequency range 5 Hz–13 MHz. The impedance response in both cases could be resolved into two contributions, associated with the bulk (grains) and the grain boundaries. An analysis by means of the ac conductivity power law showed evidence of a critical temperature of 132 and 536 °C, for the ferrimagnetic and the ferroelectric samples, respectively, which corresponds to the Curie temperature for each type of material. These results are interpreted in terms of the disorder increase approaching the phase transition. © 1998 American Institute of Physics.