High-intensity ultrasound has been applied to the ring-opening polymerisation of delta-valerolactone and epsilon-caprolactone catalysed by dibutyl tin dilaurate. Sonication was found to accelerate the polymerisation. In the case of delta-valerolactone, sonication also promoted a depolymerisation reaction so that the molecular weight fell during later stages of the reaction. Co-polymers of the two monomers were synthesised and the use of ultrasound led to preferential incorporation of epsilon-caprolactone into the material, probably due to degradation of the valerolactone sequences. (C) 2002 Elsevier Science Ltd. All rights reserved.