The formation of ZrO2:8 m01% Y2O3 solid solution was studied by electrochemical impedance spectroscopy and X-ray diffraction analysis. The basic idea, starting from monoclinic zirconia and cubic yttria, was to follow the high-temperature substitution of Y3+ ions for Zr4+ ions in the zirconia matrix by determining the zirconia cubic phase content owing to X-ray diffraction and also the intragranular (bulk) electrical resistivity due to the increase of the concentration of oxygen ion vacancies. The cubic phase volume fraction and the bulk resistivity of the ZrO2+Y2O3 polycrystalline specimens were found to depend on the square root of the time the specimen is kept at temperatures in the studied range: 1350-1450 degreesC. These results suggest a diffusion-controlled process during solid solution formation, allowing for the evaluation, by the impedance spectroscopy technique, of the thermal activation energy for diffusion of the slowest species (Zr4+), in good agreement with the determination by other techniques.