The electrical conductivity of phyllite (measured in situ at 0.5-2.5 GPa and 773-1173 K) increases with increasing temperature, satisfying an Arrhenius relation. Dehydration of phyllite at 973-1173 K enormously enhances its electrical conductivity, and the activation enthalpy (0.64-0.81 eV) remains almost constant before and after dehydration. The inflection point of the relationship between electrical conductivity and temperature is used to determine the dehydration temperature (T (d) ) at each considered pressure (P), leading to the following relationship: T (d) = 1181 - 100P. The derived relation implies that the dehydration depths of hot and cold subduction zones are similar to 70 and similar to 129 km respectively, which are both close to the depths of arc magma source regions, thereby indicating that the dehydration of pelite significantly influences the generation of melt in subduction zones.