Polystyrene latex samples have been synthesised from an oil-in-water (o/w) emulsion of the monomer using ultrasonic irradiation as the initiation source. No added chemical initiators were used in any of the reactions. The free radicals formed from the cavitation process under the action of the ultrasound are sufficient to initiate the polymerisation. In the current study, the effects of varying the input intensity of the ultrasound were investigated. A marked increase in the monomer conversion rate as a function of time was seen as the power was increased. Despite this increase in conversion rate, the increasing intensity did not affect the resultant latex particle sizes. In all cases, the particle diameters at the conclusion of a polymerisation were seen to be in the range of 40 to 50 nm. The effects of changing the concentration of surfactant in the polymerisation medium and of changing the temperature were also probed. Increases in both were seen to lead to an increased rate of conversion at a fixed input intensity. Again, the particle diameters were invariant as a function of changing conditions. The small particle sizes and high conversion rates are thought to be due to a continuous nucleation process that is caused by the scavenging of free radicals throughout the polymerisation by freshly formed small oil droplets. Copyright. 2000 Elsevier Science B.V. All rights reserved.