The well-defined monodisperse nanocrystals of pyrochlore-type antimonic acid, HSbO3-nH(2)O (n=0.5), and its mobium-substituted materials. HSb1-xNbxO3• nH(2)O (0 ≤ x ≤ 0-09, n=0.5), were successfully prepared by solution processing using the direct reaction of an aqueous H2O2 solution with antimony and niobiurn alkoxides, Sb(O-iso-C3H7)(3) and Nb(O-iso-C3H7)(5). It was found that the crystal sizes of these materials can be controlled from 35 to 50nm with intervals of a few nanometers by changing the initial Sb(O-iso-C3H7)(3) concentration. The proton conductivity of the materials is discussed in relation to their structural properties. © 2004 Elsevier Ltd. All rights reserved.