Anatase and rutile TiO2 nanoparticles were synthesized via mechanochemical reaction and subsequent annealing of the products. TiO2 nanoparticles were prepared by the use of planetary ball milling of titanyl sulphate (TiOSO(4)xH(2)O center dot yH(2)SO(4)) and NaCl powders as the reactant and diluent phases, respectively. In this paper, the effect of volume fraction of diluent phase (NaCl) on the particle size distribution and agglomeration of TiO2 nanoparticles is studied. Final products were obtained by annealing the milled powders at 700 degrees C for half an hour and subsequent washing out the water-soluble NaCl. X-ray Diffraction (XRD), Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM) investigations showed that the increase in NaCl:TiOSO4 weight ratio (NTR) leads to the formation of nanocrystalline anatase and rutile particles with more uniform size distribution, lower weight ratio of rutile phase and lower agglomeration of particles. (c) 2008 Elsevier B.V. All rights reserved.