The direct enzymatic polymerization of lactonic sophorolipids (SLs) was investigated with four lipases, including porcine pancreatic lipase (PPL), immobilized Mucor miehei lipase (MML), lyophilized Candida antarctica lipase (Fraction B, CAL-B), and lyophilized Pseudomonas sp. lipase (PSL). Several organic solvents, covering a wide range of polarity, were compared for suitability as the reaction medium. Isopropyl ether and toluene were found most effective. According to the quantification and structure identification by HPLC and LC-MS, the reaction proceeded with the formation of monoacetylated lactonic SLs and the subsequent conversion of the intermediates to oligomers and polymers, presumably through ring-opening polymerization. Temperature was found to have significant effects on the reaction. Both the conversion of reactant SLs and the subsequent formation of oligomers and polymers from the intermediates were faster at 60degreesC than at 50degreesC. The substrate selectivity among the three dominant reactant SLs also differed with the temperature. The conversion rate increased with the ring size of the lactones at 60degreesC, but it decreased with the size at 50degreesC.