Yttria (3 mol %)-zirconia powders were crystallized under hydrothermal conditions at 110 °C for 7 days from mixtures of zirconia gel with different Y-based precursors. Zirconia xerogels mixed with crystalline Y2O3 or Y(OH)3 or coprecipitated with Y(OH)3 gel were hydrothermally treated in the presence of solutions of different concentrations of (KOH + K2CO3) mineralizer. The effects of both the Y-based precursor and the concentration of the mineralizer solution on the crystallization-stabilization, the reactivity, the average crystallite size, and the degree of agglomeration of the corresponding hydrothermally synthesized ZrO2 powders are discussed.