Aqueous mixtures of either zirconium acetate or zirconium nitrate and magnesium nitrate were dried and subsequently pyrolyzed at fast heating rates (upquenching) to form metastable crystalline phases of ZrO2with various degrees of MgO supersaturation. The crystallization temperature was determined to be 380°C for the zirconium acetate, and 270°C for the zirconium nitrate at a heating rate of 5°C/min. The crystalline structures were characterized as a function of MgO content and thermal history for specimens containing 0 to 30 mol% MgO. Upquenching to 900°C, where monoclinic (m) ZrO2and MgO are the equilibrium phases, yielded single‐phase tetragonal (t) ZrO2(17 mol% MgO). The composition for whichT0(t/c) = 900°C was estimated as 9 ± 1 mol% MgO. Compositions crystallizing as metastablet‐ZrO2(<8 mol% MgO) partitioned at higher temperatures and/or longer times into two‐phase mixtures, following the general sequencet→t+m→m+ MgO. Similarly, compositions forming metastablec‐ZrO2(10 to 30 mol% MgO) partitioned in the following sequence:c→c+t+ MgO →t+ MgO →t+m+ Mgo →m+ Mgo. The initial phase selection and subsequent partitioning sequence are discussed in light of phase hierarchies predicted from thermodynamic concepts and kinetic constraints which are introduced by the solute partitioning