Tetragonal (t) ZrO2 solid solutions were prepared with additions of Nb2O5 up to 1.5 mol% to 3 mol% Y2O3-stabilized t-ZrO2 (3Y-TZP). Aging of the solid solutions at 70-500 degrees C in air induced not only the transformation of t-ZrO2 to monoclinic (m) phase but also to rhombohedral (r) ZrO2. The amount of t --> m transformation increased with increasing Nb2O5 content and grain size but the activation enthalpy related to the low temperature degradation was not varied by the parameters. The rate of low temperature degradation depended on the number of oxygen vacancies and the internal strain energy, both of which were determined by the pentavalent cation content.