A peroxo niobic acid sol was prepared by peptization of the niobic acid precipitate (Nb2O5.nH(2)O) with a H2O2 aqueous solution. Crystallized Nb2O5 nanoparticles and niobic acid nanoparticles were obtained by heating the peroxo niobic acid sol. When peroxo niobic acid sol prepared by peptization of the niobic acid precipitate ([NH3] = 0.3 mol/l) was heated at 348 K for 1 week, Nb2O5 nanoparticles with a diameter of 4.5 nm and a SBET of 275 m(2)/g were obtained. When peroxo niobic acid sol prepared by peptization of the niobic acid precipitate ([NH3] = 1 mol/l) was heated at 348 K for 1 week, niobic acid nanoparticles with a diameter of less than 2 nm were obtained. The pore structure and degree of crystallinity of the nanoparticles prepared by heating the peroxo niobic acid sol greatly depended on the concentration of the ammonia solution used for preparing the niobic acid precipitate. (C) 2003 Elsevier Inc. All rights reserved.