The influence of temperature between R.T. and liquid nitrogen (−196°C) on the strength and toughness of three grades of Mg-PSZ and a 3 mol% Y-TZP material have been investigated. The toughness of the Mg-PSZ materials passes through a maximum at the Ms temperature, whereas the strength increases monotonically with decreasing temperature regardless of Ms. The Y-TZP material exhibits increasing toughness with decreasing temperature but the strength passes through a maximum at -80°C. The results are discussed in terms of transformation and R-curve limited strength of transformation toughened ceramics. For Mg-PSZ materials it is suggested that the critical stress to initiate a non reversible transformation on the tensile surface is responsible for microcracking and ensueing R-curve development.