The three polymorphs of titania (anatase, rutile and brookite) have been obtained as nanoparticles selectively and with well definite morphologies (platelets of brookite, rods of rutile) by thermohydrolysis of TiCl4 in concentrated aqueous nitric acid. The selectivity of the synthesis depends strongly on the acidity of the medium. The presence of concentrated nitrate ions seems to be the determining factor for the formation of brookite and its stabilization against recrystallization.