Direct numerical simulation of the formation and growth of titanium dioxide nanoparticles in two-dimensional, temporal mixing layers is performed. Titania is produced by the gas-phase hydrolysis of titanium tetrachloride at a temperature of 300 K. The flow field is obtained by solving the Navier-Stokes equations and evolution of the particle field is obtained via a nodal method. The approach approximates the aerosol general dynamic equation and is advantageous in that there are no a priori assumptions regarding the nature of the particle size distribution. The formation and growth of particles up to and including 128 nm in diameter in iso-thermal flows are studied. Simulations are performed for two initial reactant concentration levels. The evolution of the particle field as a function of space, time and size is presented. Results indicate that particle formation and growth is mixing limited and particles grow faster with increasing initial reactant levels. Additionally, the results indicate rather large geometric standard deviations that vary significantly throughout the computational domain. (C) 2005 Elsevier Ltd. All rights reserved.