A major component of the driving force for crystallization of hydrous zirconia-yttria powders comes from their noncrystallinity. Subsequent to dehydration of coprecipitated gels, the amorphous-to-crystalline phase transformation is the regime of primary crystallization. For the present study of 3.0-, 5.0-, and 9.0-mol%-yttria-stabilized zirconia, the powders were produced as fine aggregates of similar to300 nm median diameter. The activation energy in all three cases was determined by the hot-stage X-ray diffraction technique where the powders were heated isothermally in the temperature range of 500degrees-600degreesC. The Avrami equation was used to calculate the rate constant value and the Arrhenius relationship was utilized to determine the value of the activation energy for primary crystallization.