The reactivity of mono- and multi-coordinated hydroxyl groups on Pt/ZrO2 with CO was investigated with in-situ transmission FT-IR. It was found that mono-coordinated hydroxyl groups on the ZrO2 support reacted with CO to formate. Multi-coordinated hydroxyl groups were needed for formate decomposition, producing H-2 and CO2, taking place exclusively in the presence of Pt. The observed differences in reactivity of different types of OH groups are relevant for the WGS mechanism, assuming that formate is a reactive intermediate under the conditions used in this study (300-400 degrees C). The fact that Pt is needed for decomposition of formate indicates that the reactivity of formate depends on the position on the ZrO2 surface, i.e. in close vicinity or remote from metal particles. Our results show that reduction of the support was limited to structural defective zirconia sites at edges and kinks, hosting the mono-coordinated hydroxyl groups. These sites can be re-oxidized with water, re-establishing the hydroxyl groups. (C) 2008 Elsevier Inc. All rights reserved.