Powders of polyethylene, polypropylene, poly(vinyl chloride) and poly(methyl methacrylate) have been subjected to irradiation with high-intensity ultrasound while suspended in water. Changes in the particle sizes and the surface morphology were noted and the pattern of the results was correlated with the physical properties of the materials. In anticipation of sonochemically enhanced reactions at polymer surfaces, the effect on a polyethylene sheet was also examined. The implications of the results for preparative methods as well as surface reactions are discussed.