A series of tantala-silica catalysts (15 wt.% Ta2O5) were prepared by sol-gel from tantalum and silica ethoxides. These syntheses were carried out using two families of quaternary ammonium salts, CnH2n+1(CH3)(3)NBr (n = 14, 16, 18) and (CnH2n+1)(4)NBr (n = 10, 12, 16, 18) were used as surfactants. The catalysts were characterized by adsorption-desorption isotherms of N-2 at 77 K, XRD, SAXS, DRUV-Vis, py and CDCl3-FTIR, and XPS. The catalytic tests were carried out in dioxane, methanol or CHCl3, and indicated that both conversion and selectivity depend on the size of tantalum nanoxide. Leaching investigation showed that part of Ta is extracted from the network, but it segregates on the support as Ta2O5. (C) 2004 Elsevier B.V. All rights reserved.