Catalysts containing from 8 to 28% wt. of niobia supported on gamma-alumina by impregnation with mobium pentaethoxide solutions were characterized by infrared spectroscopy of adsorbed pyridine as a probe for acidic sites and CO2 adsorption as a probe for basic sites. The catalysts had their activity measured in isopropanol dehydration at 453 K, 1-butene isomerization at 348 K and in cumene dealkylation at 728 K. The density and strength of the Lewis acid sites and of the basic sites decreased with niobium content. On the other hand, the density of Bronsted acid sites increased in the same direction. Different reactions responded differently to mobium addition. In isopropanol dehydration, the main factor responsible for the observed decrease in catalytic activity was the decrease in concentration of basic sites. Activity for 1-butene isomerization also decreased with mobium addition, but the main factor seemed to be the decrease in concentration of alumina-associated Lewis acidic sites. Evidence from cis-Itrans-2-butene ratio indicated that niobium addition modifies the properties of neighboring aluminum sites. Bronsted acidic sites created by mobium addition are responsible for the development of activity in the cumene dealkylation reaction, but the sites associated with tridimensional niobia species seem to be more effective in this reaction. (c) 2006 Elsevier B.V. All rights reserved.