Transparent lead lanthanum zirconate titanate (PLZT) ceramics were investigated by ac impedance spectroscopy at temperatures between 300 and 500 degrees C under different oxygen partial pressures with the purpose of addressing the role of boundary effects on the electrical conduction properties of this compound. It is found that in spite of the high lanthanum content, holes are the dominating electronic charge carriers leading to p-type conductivity in oxidizing conditions. The grain boundaries reveal a blocking behavior (holes depletion), which is explained in the framework of the space charge concept. A positive space charge potential of 0.23 V is obtained. (C) 2011 Elsevier B.V. All rights reserved.