The surface properties of ZrO2 in He, H-2, O-2, air and wet air at temperatures ranging from 25 to 600 degreesC have been investigated by means of ESR method. The Zr3+ centers characterized by ESR can be described as the oxygen coordinatively unsaturated Zr sites, mainly located at the corner sites of ZrO2 microcrystallites, on the surface of ZrO2. More Zr3+ centers were formed on the surface of ZrO2 by treating the sample in He at temperatures above 400 degreesC. And the formation of the Zr3+ centers may be associated with the removal of surface hydroxyl. Although Zr4+ in ZrO2 cannot be reduced at temperatures of up to 600 degreesC in H-2, some surface hydroxyl can be reduced to form F-centers at temperatures higher than 250 degreesC. And O-2(-) can be produced from the Zr3+ centers and F-centers at temperatures above 400 degreesC, and it may be associated with both H-2 and the trace amount of O-2 in H-2. (C) 2003 Published by Elsevier B.V.