Zirconia, pure and modified by sulfate or molybdate was prepared in a molten nitrate medium. Sintering, crystallization and the tetragonal–monoclinic (T–M) phase transition were studied by using differential thermal analysis (DTA), X-ray diffraction (XRD) and surface area measurements. Doping of the reaction mixture with oxoanions such as pyrosulfate and heptamolybdate changes the morphology of samples, increases their specific surface and stabilizes the T variety of zirconia. Oxoanions also improve the stability to calcination in air. It was suggested on the base of thermal analysis data, that oxoanions are included in the amorphous phase which is spread around the crystallites of zirconia. This amorphous phase induces stabilization of the T form of ZrO2 and causes the specimens to have a high surface area. This stabilizing effect persists up to the temperature of crystallization of the amorphous phase (610 and 800 °C for sulfate- and molybdate-doped systems, respectively). The last process is accompanied by an exothermic peak in the DTA curve and subsequent sublimation of the corresponding oxide (SO3, MoO3).