This work describes the aging of aqueous colloidal TiO2 nanoparticles at different temperatures and pH values. Emphasis was particularly given to the behaviour in the alkaline region, since this has not yet been properly reported. Aging of colloidal TiO2 suspensions has previously been investigated using spectroscopic methods (band gap energy determination) and a dynamic light scattering method (particle size determination). The photocatalytic activity, representing the most important property of TiO2 nanoparticles, was determined through the quantum yield of 4-chlorophenol degradation. It was found that the size of acidic colloidal TiO2 particles depends on the storing temperature; lower temperature leads to a small increase in particle size. The process of aging of TiO2 nanoparticles consists in slow re-crystallization that leads to slightly larger particles, and it seems that this process is faster at higher temperatures. In alkaline environments it was found that the change in particle size depends on the type of alkaline reagent used (KOH and NH4OH), on its concentration and also on the starting concentration of the TiO2 suspension. In contrast to the quantum yield of acidic TiO2, the quantum yield of alkaline suspensions decreased during aging. (C) 2010 Elsevier B.V. All rights reserved.