The effects of water vapor on the oxidative dehydrogenation of isobutyric acid to methacrylic acid over an iron phosphate catalyst were studied. The presence of water vapor suppresses both the carbon deposit and the formation of phosphorus-rich species on the surface of the catalyst and, as a result, it prevents the degradation in both the catalytic activity and the selectivity which occurs during the reaction, though it has no clear influence on both the redox properties and the structure of the catalyst. The pretreatment of the catalyst with a gas containing water vapor or the reaction in the presence of water vapor restores the activity and selectivity of the degraded catalyst. It is proposed that water plays a role in removing the carbon and oxygenated products from the surface, in suppressing the side reaction, that is, the decomposition of isobutyric acid to propylene and CO, and also in modifying the surface species.