Substitution of TiO2 for ZrO2 into single-phase tetragonal (t') 7YSZ has been shown to induce a twofold increase in toughness. This enhancement has been achieved by increasing the tetragonality of the unit cell, upon substituting Ti4+ for the larger Zr4+ cation. The observed behavior is consistent with ferroelastic toughening, but direct evidence of the mechanism is lacking. Adding TiO2 to 7YSZ has the additional benefit that it diminishes the transformability of the depleted tetragonal form: specifically, no monoclinic phase was observed even after equilibration at 1600 degrees C, followed by low-temperature annealing. The combination of properties bodes well for potential application as a thermal barrier coating in gas turbine engines.