In order to increase molecular weight of PLGA, BuOH was introduced as a co-initiator to Sn(Oct)2 in ring-opening precipitation polymerization of L-lactide and glycolide in supercritical carbon dioxide (ScCO2). The polymerization mechanism of the reaction processes on Sn(Oct)2 with BuOH was explored. The positive effect of BuOH on polymer molecular weight was studied. PLGA with higher Mw (of 19,500 g/mol) and narrower PDI (of 1.12) was successfully prepared by ring-opening precipitation polymerization in ScCO2 using Sn(Oct)2/BuOH as an efficient catalytic system. Influence of reaction conditions, such as monomer feed concentration, reaction temperature, pressure, and time, were also investigated in detail. POLYM. ENG. SCI., 54:704–710, 2014. © 2013 Society of Plastics Engineers