High temperature mechanical properties of zirconia reinforced with carbon-nanotubes (CNTs) are studied by mechanical spectroscopy. CNTs limit zirconia grain-boundary sliding (GBS) and consequently deformation. Concurrent grain-growth and plastic deformation lead to a substantial error in the evaluation of deformation activation-enthalpy. Hence, a new calculation was developed in order to account for the effect of grain growth. The corrected enthalpy is comparable with the lattice diffusion energy of cations (Zr4+, Y3+), which evidences that at low-stress regime, GBS is accommodated by diffusional creep. (C) 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim