First measurements of oxygen grain boundary diffusion coefficients in nanocrystalline yttria-doped ZrO(2) (n-ZrO(2).6.9 mol % Y(2)O(3)) are presented. The (18)O diffusion profiles measured by secondary ion mass spectroscopy are much deeper in the nanocrystalline specimens than in single crystals. An oxygen diffusivity, D(B), in the grain boundaries can be deduced, which is approximately 3 orders of magnitude higher than in single crystals. From the present data the temperature variation of the oxygen grain boundary diffusivity, D(B) = 2.0 x 10(-5) exp (-0.91 eVk(B)T) m(2)s, and the oxygen surface exchange coefficient, k = 1.4 x 10(-2) exp (-1.13 eVk(B)T) ms, are derived.