Water minimized a decrease in conversion for the isomerization of linear butene to isobutene on silica-supported heteropolyacid (H4SiW12O40) catalysts while maintaining high selectivity toward isobutene. The role of water was related to surface acidic properties (number of surface acid sites and acid strength) and characteristics of hydrocarbon deposits (extent of deposits and hydrogen/carbon ratio), as determined by benzonitrile and ammonia temperature-programmed desorption as well as temperature-programmed oxidation of fresh and spent catalysts. It appears that highly unsaturated hydrocarbon deposits consisting of 8-12 carbon units covered surface acid sites, especially strong acid sites, limiting the isomerization reaction. Water in the feed was helpful in preserving the acid sites and decreasing the number of carbon units in the hydrocarbon deposits. Thus, conversion occurred more readily in the presence of water than in the absence of water. (C) 2008 Elsevier Inc. All rights reserved.