The acidic and catalytic properties of a series of silicon-tantalum mixed oxides containing between 0 and 30 at% tantalum were characterized by temperature-programmed desorption of ammonia, by FTIR spectra of adsorbed pyridine, and by the test reaction of 1-butanol dehydration at 250-300 degrees C. Probe molecule adsorption and catalytic testing show an acid site generation in silicon-tantalum mixed oxides compared to pure tantalum oxide. Both Bronsted and Lewis acid sites are present on the mixed oxide surface. Bronsted and Lewis acid sites seem to be weaker in SiO2-Ta2O5 mixed oxide than in analogous SiO2-Al2O3, since a greater reaction temperature is required to achieve the same conversion in the above test reaction. However, a great advantage of this new solid acid is the production of butenes with 100% selectivity.