The objective of this study was to examine the reactivity of superoxide ion as an oxygen nucleophile towards biodegradable biomaterials having an aliphatic polyester structure, such as poly(D,L-Lactide) (PDLLA). The changes in molecular weights and thermal properties of PDLLA were studied as a function of the superoxide ion concentration, hydrolysis time, and hydrolysis temperature. The superoxide ion induced fragmentation of PDLLA yielded a mixture of various species with different chain length. A combined size exclusion chromatography (SEC) method with a chemical tagging by phenyl isocyanate revealed that the structure of the oligomer species with the lowest molecular weight formed in the superoxide ion induced degradation mixture was linear. The significant reduction in T-g was consistent with the significant reduction in the molecular weight; of PDLLA, The linear low molecular fragments (dimers and trimers) in the reaction mixture could act as an internal plasticizers to enhance the T-g reduction by increasing the free volume of chain ends. The relationship of T-g with molecular weight of PDLLA followed the general theory of Fox-Flory. The mechanism of simple hydrolysis of ester by superoxide ion proposed by Forrester et al. was modified to interpret the data obtained from the synthetic biodegradable polymers.