Lactones are efficiently oligomerized by heteropolyacids H-3[PM12O40](.)aq with M=Mo or W, "H3+n[PMo12-nVnO40](.)aq" (HPA-n) and molybdenum(VI) acetylacetonate, vanadium(V) sulfate initiators in the presence of dioxygen. The resulting linear oligomers display relatively low polydispersity (<= 1.7) and high monomer conversion (>= 99 %), and are obtained with relatively short reaction times (1-24 h, 20-60 degrees C). The products were analysed by various physicochemical methods: size-exclusion chromatography (SEC) (to determine the molecular weight properties); IR; H-1 and C-13 NMR; matrix-assisted laser desorption ionization-time-of-flight mass spectrometry (MALDI-TOFMS). Lactone oligomerization is proposed to proceed by a coordination-insertion mechanism via cationic species (VO2+ and/or MO22+; M=Mo or W) with chain growth involving acyl-oxygen bond cleavage and an activated monomer. Bronsted acids catalyse the reaction. These reactions are the first examples of the ring-opening oligomerization of lactones catalysed by heteropolyacids and the inexpensive "VOSO(4)(.)alpha H2O/THF-H2O/O-2" system. (c) 2005 Elsevier B.V All rights reserved.