The effect of ultrasound on the ring opening polymerization of octamethylcyclotetrasiloxane to give poly(dimethyl siloxane), PDMS, is described. Sonication was found to increase the rate of polymerization over conventional stirring and to give higher molecular weights with lower polydispersities. The results are explained in terms of the efficient mixing and dispersal of acid catalyst given by sonication. The basic effects of sonicating dilute PDMS solutions are also described but it was found that, in contrast to other polymerizations, the degradation plays little part in determining the molecular weight during synthesis.