The possibility of using acoustic spectroscopy for the on-line and in-situ monitoring of particle size distributions and volume concentrations in batch crystallization processes is investigated. First, the acoustic technique is discussed, along with the evaluation of the involved model parameters. Second, for a seeded cooling crystallization of potassium sulfate (K2SO4) in a supersaturated aqueous solution, it is shown that both size and concentration of the growing crystals can be estimated simultaneously from measured acoustic attenuation spectra. The results are discussed in view of the potential application of this technique to real-time and in-situ monitoring and control of industrial processes.