Abstract The electrical impedances of glasses in the system BaO ∗ Al 2O 3 ∗ B 2O 3 were studied in the temperature range from 550 to 1150 °C using different heating and cooling rates. The specific conductivities were illustrated by fitting Arrhenius functions to the data and the derivative (−d ln σ/d(1/ kT)). In this system, the activation energy of the electrical conductivity depends on temperature. Crystallization of the melt affects the derivative. Depending on the specific conductivities and the respective activation energies of the liquid and the crystalline phase, maxima or minima (or both) are observed in the derivative. The melting of the crystalline phase during heating results in a maximum of the derivative. The shape of the derivative can be explained taking into account the specific conductivities and the attributed activation energies of the liquid and crystalline phase.