The gas-phase hydrolysis of TiCl4 has been studied with several laboratory reactors. Unexpectedly, the reaction is clearly split in two steps, an almost instantaneous partial hydrolysis which produces 2 mol of HCl/mol of TiCl4 together with a salt, identified as Ti(OH)(2)Cl-2. The latter forms a very fine dust which easily evolves into aerosols. Ti(OH)(2)Cl-2 in a moist environment eventually hydrolyses to HCl. The kinetics of both steps have been studied. While the first step is very fast, the salt hydrolysis is much slower, its kinetics being controlled by diffusion of the gaseous reactant through the shell of the solid product covering the aerosol particles.