The understanding of water effects on solid acid catalysts is a key issue in developing efficient processes to produce olefins by bio-alcohols dehydration. In this work, the effects of water on TiO2/SiO2 catalysts for the gas phase conversion of isobutanol into linear olefins have been unraveled, using for the first time in situ acidity measurements achieved with a flowing NH3 probe and water vapor containing gas mixtures at adsorption equilibrium in the temperature range of the reaction. Such compounds, prepared by grafting titania onto mesoporous silica, contain well-dispersed TiO2 amorphous clusters anchored by Ti-O-Si linkages, leading to much higher catalytic activity than TiO2 and SiO2. They yielded only dehydration products, among which 30% were linear butenes. Furthermore, their activity was significantly improved by addition of water into the feed, whatever the contact time. Acidity measurements indicated that TiO2/SiO2 mixed oxides were mostly of Lewis type after activation at 450 degrees C. However, in situ FTIR acidity measurements showed that addition of H2O to NH3/He gas mixture has no influence on the number of Lewis sites, while weak Bronsted sites were formed on other sites. This formation, due to a shift of equilibrium depending on both the temperature and the H2O partial pressure, corresponds to the hydrolysis of Ti-O-Si bonds, generating OH acidic groups and enhancing catalytic activity under water vapor. The novel type of acidity measurements used in this work appear powerful and can be applied to other acidic heterogeneous catalysts. (C) 2016 Elsevier Inc. All rights reserved.