Precipitation of concentrated titanyl sulphate is an important step in the Sulphate Process used for manufacturing titanium white in which titanium dioxide particles are formed and the particles size as well as its distribution is determined. This study investigated the transient changes of TiO2 precipitation process by using two on-line particle measurement techniques, turbidimeter and focused beam reflectance measurement (FBRM), in order to explore the mechanism of particles formation. Induction period as well as primary nuclei formation of the precipitation process was calculated by turbidity value changes and the influence of feeding rate on primary nucleation, primary crystal sizes and final product particle size distribution (PSD) were also investigated. FBRM was employed for detecting the transient variations of PSD during the process. It was found that the process was divided into four stages: induction, rapid hydrolysis and aggregation, aggregation balance with further reaction, and ripening. The Sigmoidal-Boltzmann model is used to describe these transient phenomena and describes well the formation of TiO2 particles. (C) 2010 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.