Cyclic oligomers of polyester show great potential for a reaction-injection-molding process, because of their initial low viscosity and rapid ring-opening polymerization at low temperatures (180 degrees C) without exothermic reaction or condensates. In this work, we report the synthesis of cyclic oligo(butylene terephthalate) (COBT) from linear poly(butylene terephthalate) by a formation-extraction process employing supercritical fluids (SCF) CO2, and pentane at T = 230 degrees C and P = 250 bar. Following this, depressurization of SCF leads to easy recovery of the COBTs. When compared with SCF CO2, SCF pentane is found to be an attractive solvent because of its higher solubilizing capacity (0.8 mg COBT dimer/g pentane) for the COBTs. (c) 2006 Wiley Periodicals, Inc.