The influence of different humidity levels on the tribological behaviour of polycrystalline tetragonal zirconia polycrystals (Y-TZP) sliding against commercial Y-TZP balls has been investigated. Fretting experiments (mode I) have been conducted at three different relative humidity (RH) levels, namely 5%, 50% and 85% RH. Based on the obtained wear and friction data as well as the morphological investigation of the worn surfaces, the wear mechanisms in different humidity conditions are proposed. The morphology of the wear pits suggests that plastic deformation coupled with delamination is the wear mechanism in dry sliding (5% RH) and in ambient humidity (50% RH) conditions. Additionally, Raman measurements showed that the phase transformation of tetragonal zirconia phase to monoclinic symmetry also plays a role in the fretting wear under all the investigated humidity conditions. XPS analysis of the wear debris formed at 85% RH indicates that tribochemical wear is the dominant wear mechanism at high humidity conditions. (C) 2000 Acta Metallurgica Inc. Published by Elsevier Science Ltd. Ail rights reserved.