The superplastic characteristics in 3 mol% yttria doped tetragonal zirconia polycrystals (TZP) were drastically improved by the combined addition of germanium oxide and other dopants (titanium oxide, magnesium oxide, and calcium oxide) giving rise to 1000% elongation under optimum conditions. Extensive work hardening of the co-doped compositions for temperatures above 1400 degreesC was observed. These were unique features which could not be explained from the work hardening due to dynamic grain growth. Rather it is proposed that the metastable nature of these co-doped materials, which changes during deformation is the cause of this exaggerated work hardening.