In this work titanium dioxide nanoparticle formation and evolution via the sol-gel route is investigated. Alcoholic solutions of titanium tetra-isopropoxide and water are mixed in a Vortex Reactor, where the mixing time can be accurately controlled by manipulating the inlet flow rates. Particle synthesis is carried out in operating conditions ranging from poor to excellent mixing performances and the effect of mixing on the formation of titanium dioxide nanoparticles and on their evolution is investigated from the modelling and the experimental view points. Experiments are in fact interpreted by using a simple population balance model and mixing in the reactor is accurately described by a computational fluid dynamics model based on the calculation of characteristics mixing times. Results show that particle formation is strongly influenced by mixing and that particle aggregation/condensation occurs in the reaction limited regime. (C) 2008 Elsevier B.V. All rights reserved.