Binary TiO2-SiO2 mixed oxides display interesting catalytic properties which have not been attributed to the pure TiO2 and SiO2. Many authors have proposed that Brphinsted acidity is generated upon interaction of the two oxides. Recent results from our laboratories have demonstrated that high-purity crystalline and amorphous TiO2-SiO2 mixed oxides do not possess Brphinsted acidity, only Lewis acidity. We have investigated the catalytic properties for 1-butene isomerization up to 340 degreesC. High-purity TiO2, prepared with low temperature supercritical drying, has a high catalytic activity, close to that of the TiO2-SiO2. The reaction products obtained from TiO2 and from TiO2-SiO2 are identical and consist of linear butenes only, while Brphinsted acid catalysts give rise to isobutylene and C-3-C-5 hydrocarbons. These results suggest that nanoparticles of TiO2 stabilized by SiO2 are the active components of TiO2-SiO2. No evidence for Brphinsted acidity in TiO2-SiO2 has been found. (C) 2003 Elsevier Inc. All rights reserved.