The in vitro hydrolytic degradation of thin (ca. 100 mum) films of poly(glycolic acid) (PGA) has been studied, focusing on the kinetics of degradation of the near surface phases. The emphasis of this work is to describe the nature of the initial "induction" period of hydrolysis of biodegradable polyesters, where water absorption is initiating into the surface phase and weight loss is not detectable. The kinetics of surface degradation are shown to be related to the enrichment of amorphous PGA in the surface phases. This study presents results from both surface and bulk phase characterization of short-term (< 10 h) exposures of PGA to buffered systems at physiological temperatures. The role of pH of the external buffer medium in influencing the rate of PGA hydrolytic surface degradation is quantified using time-of-flight secondary ion mass spectrometry (ToF SIMS), using previously developed methods to determine the rates of production of oligomeric reaction products. Scanning electron microscopy (SEM) studies show changes in surface morphology over these exposure periods. X-ray photoelectron spectroscopy (XPS) data are given to support the chemistry of the process. Finally, DSC data on the changes in bulk crystallinity and weight loss studies confirm the nature of the early stages of the reaction. Results show that the relative hydrolytic surface degradation rates in pH 4.0 and 10.0 buffer media are approximately 3 times faster than that in pH 7.4 buffer, but the change in crystallinity at pH 4.0 is less than that at pH 10.0. SEM results show the emergence of increased surface crystallinity at the more extended exposure times in this study (6 h). No detectable weight loss is reported for these exposures. A model of the reaction chemistry of the induction period is presented which involves the initial rapid, ps-dependent hydrolysis of surface segregated amorphous PGA, with little overall weight loss as water absorption is still reaching equilibrium in the reaction zone. These results indicate that the reaction kinetics during the initial induction period could lead to rapid drug delivery from the surface with little release of monomeric glycolic acid upon initiation of reaction.