Nanocrystalline powders of anatase and rutile type of titania with different morphology and particle size 13-50 nm (XRD) were prepared by a hydrothermal treatment of complex titanyl oxalate acid H2TiO(C2O4)(2) (0.28 and 0.07 M) aqueous solutions. The influences of hydrothermal treatment conditions on the formation features, phase composition, particle size, morphology and properties of the products were investigated and discussed. Under the conditions of high temperature hydrolysis of 0.28 M H2TiO(C2O4)(2) aqueous solution, the formation of mesoporous anatase particles (60-100 nm, TEM) containing non-intersecting (7-27 nm, TEM) pores with encapsulated solution was observed while hydrothermal treatment of 0.07 M aqueous solutions did not lead to the formation of mesoporous TiO2. The formation mechanism of mesoporous anatase particles was suggested considering the process as recrystallization primary grains aggregates. (C) 2004 Elsevier B.V. All rights reserved.