The microstructure and superplastic deformation of fine-grained undoped 8Y-CSZ and 1 wt% CuO doped 8Y-CSZ have been investigated in tension in the temperature range 1503 to 1623 K and strain rate range 5 x 10(-5) to 10(-3) s(-1). Deformation of the undoped 8Y-CSZ was characterized by large strain-hardening with limited tensile elongations of 20%; this was mainly due to severe grain growth during deformation. The addition of a small amount of a transition metal oxide (CuO) resulted in a decrease in strain-hardening and enhanced tensile elongations up to 78%. The ductility enhancement in the CuO doped 8Y-CSZ was due to copper segregation to grain boundaries, th us facilitating grain boundary sliding. In addition, the enhanced ductility in the doped material was related to a reduction in flow stress which, in turn, suppressed cavitation and delayed fracture. (C) 1998 Kluwer Academic Publishers.