In fine-grained zirconia, the order of stability of the three polymorphs is exactly the reverse of that observed in a coarse-grained sample. Starting from a disorder amorphous material, the crystallization into the cubic form occurs, in general, below 500 degreesC. It is followed, or accompanied, by the formation of tetragonal ZrO2 and, finally, by the crystallization into monoclinic zirconia above 700 degreesC. This sequence is observed in mesoporous and microporous zirconia. It has been suggested that this anomalous behavior is due to a high surface energy of the monoclinic phase or to strains present in the small particles. It is shown here that the surface effect explains satisfactorily and semiquantitatively the observations. This does not rule out other factors.