Oxygen ionic conductivity through zirconia (ZrO2) is essential to the performance of solid oxide fuel cells, thermal barrier coatings, and zirconium alloys for nuclear fuel cladding. Since sulfur (S) atoms can replace oxygen atoms at ZrO2 surface or even induce formation of homogeneous zirconium oxysulfide (ZrOS) structure at high S partial pressure, we study defect migration and formation in both cubic zirconia (c-ZrO2) and ZrOS under different electron and element chemical potentials using density functional theory. Our calculations show that S addition to zirconia, either by doping or through gas diffusion, increases both the formation energy and migration barrier of doubly positively charged oxygen vacancies. Since the charged oxygen vacancies play a vital role in the ionic and thermal conductivities, our results suggest that high S partial pressures are expected to change the mechanisms of ionic and thermal conductivities of ZrO2-based materials. (C) 2012 Elsevier B.V. All rights reserved.