The gel to sol transition of aqueous solutions of di- and triblock copolymers consisting of poly(ethylene oxide) and biodegradable polyesters was studied as a function of temperature. The molecular weight and the chemical composition of the biodegradable blocks, (poly(L-lactic acid), poly(DL-lactic acidi, poly(DL-lactic acid-co-caprolactone), and poly(DL-lactic acid-co-glycolic acid)) were varied to investigate the effects of chain packing and relative hydrophobicity on the gel to sol transition. The block copolymers studied formed micelles at lower concentrations in water, while the concentrated solutions experienced a gel to sol transition as the temperature increased. Further increase in temperature resulted in the precipitation of polymers. With increasing molecular weight and chain packing tendency of hydrophobic biodegradable block, the gel to sol transition occurred at lower concentrations and the transition temperature ranged from 0 degrees C to over 90 degrees C in a relatively narrow concentration range. The results obtained in this study confirm the relationship between gelation properties and polymer structure, as well as provide more information for these polymers in drug delivery applications.