Zirconia-based materials are widely investigated and used as electrolytes in solid-oxide fuel cells, oxygen sensors and electrochemical devices. These materials present polymorphism, which has a critical effect on their technologically important properties. The polymorphism is influenced by, among other factors, aliovalent dopant nature and content, grain size and interfacial energy. In this work, we investigated the crystal structure of ZrO₂-12 mol% CaO and -9 mol% Y₂O₃ dense ceramics as a function of grain size. We found that the samples undergo a phase transition from the t″ form of the tetragonal phase to the cubic phase with an increase in grain size. This transition is directly detected by Raman spectroscopy and further evidence is given by a change in the activation energy for bulk ionic conduction. The transition occurs at an average grain size greater than 500 nm for both systems.