The chemical compositions at the surface and in the bulk of the sol-gel-derived ZrO2 films calcined at elevated temperatures in air or in N2 atmospheres were examined to understand the metastability of the tetragonal phase and the mechanism of its phase transformation. The phase evolution of ZrO22 in air followed the sequence of amorphous → m-tetragonal → monoclinic over 80-950°C, while the phase transformation of amorphous → m-tetragonal → monoclinic → m-tetragonal was observed under N2 atmosphere. The reduction of Zr4+ to low-valent states and the generation of oxygen vacancies via dehydroxylation and deoxygenation play the crucial roles in stabilizing m-tetragonal ZrO 2 in the sol-gel-derived films. The O/Zr ratios for stabilizing the m-tetragonal ZrO2 ranged between 1.98 and 1.63. The m-tetragonal-to-monoclinic phase transformation primarily involves the segregation of lattice defects to grain boundaries and occupation of oxygen vacancies by the diffused O2- ions that were converted from surface hydroxyl groups. In the absence of alternative oxygen donors, including water and oxygen molecules, the stability of m-tetragonal ZrO2 was maintained at elevated temperatures under N2 atmosphere. In addition, regeneration of the oxygen vacancies via deoxygenation at high temperature results in the reformation of m-tetragonal ZrO2. The changes in the chemical compositions and crystallite sizes of the films depict that the m-tetragonal-to-monoclinic phase transformation starts from the core of tetragonal domains, while its retransformation begins from the boundaries of monoclinic grains.