The aim of this study is to analyze the behaviour of dense colloidal suspensions in flow by acoustic spectroscopy. The destabilization and the aggregation of stable colloidal silica dispersions in a stirred tank are obtained by the addition of salt. Experiments were made in order to observe the influence of different operating parameters, like silica concentration, temperature and stirring speed, on the behaviour of the suspended particles. The use of on-line acoustic spectroscopy to analyze the process enables us to evaluate the evolution of the silica suspension properties during the aggregation processes. For example, the influence of physicochemical and hydrodynamics parameters on the aggregation process can be simply explained on the basis of the acoustic attenuation spectra. Thus the direct analysis of the spectra can give information on the evolution of the aggregation process and a fast comparison of the effects of the various operational parameters. Online acoustic spectroscopy was used to evaluate the evolution of colloidal silica suspension properties during a destabilization process obtained by the addition of salt. The direct analysis of the acoustic attenuation spectra can give information on the evolution of the aggregation process and a fast comparison of the effects of the physicochemical and hydrodynamics parameters. ▪