In this communication, new results on the structure of silica-supported molybdenum oxide catalysts using operando and in situ Raman spectroscopy are presented. The detailed structure, that is, the ratio of dioxo to monooxo surface molybdenum oxide, strongly depends on temperature and gas-phase composition. Whereas increasing the catalyst temperature to 500 degrees C results in an increase in the monooxo/dioxo intensity ratio, the ratio is reduced by the presence of steam. The observed behavior is attributed to shifts in the equilibrium between monooxo and dioxo species resulting from hydration/dehydration of the catalyst in agreement with theoretical predictions. (C) 2012 Elsevier Inc. All rights reserved.