Aqueous suspensions of Na-montmorillonite (1.80 and 0.60 wt%) were aged for three months and were sonicated for different times. Statistical parameters related to volume were determined from particle-size distributions obtained by Laser Shadow Analysis. After a long ultrasound treatment the concentrated suspension showed a gradual increase of the largest particles and of the mean and median particles, as well as a decrease of the percentage of particles with diameters under 1.5 mum. The dilute suspension, on the other hand, showed a decrease of the largest particles and of the mean and median particles, and an increase of the percentage of particles with diameters under 1.5 mum. In the dilute suspension sonication resulted in the formation of small swollen particles by the delamination of large swollen tactoids. In the concentrated suspension the swelling of the clay was not complete during the three months aging period. As a result of the ultrasound treatment tactoids were delaminated and the swelling of the clay particles became more efficient. (C) 2004 Kluwer Academic Publishers.