Aliphatic polyesters are the source of the attractive polymeric matrices that are currently investigated for making controlled drug delivery devices. In spite of the large number of investigations dealing with LA/GA polymers which have been reported in the literature, only little is known about the degradation mechanism of these polyesters in solid state. In this study poly (L,D-lactide-co-glycolide) 50:50 (PLGA) was hydrolyzed at 37oC for periods up to 56 days. PLGA discs were prepared in 60oC and the results of their in vitro biodegradation behaviour are reported. We observed that hydrolysis of PLGA proceeds in 7 stages: surface hydrolysis ,outer layers hydrolysis, outer layers water soluble oligomers formation, exiting water soluble oligomers, bulk hydrolysis and exiting degradation products, bulk catalyzed hydrolysis, and at the last step fragile porous structure formation. The remained porous structure is crystallized and shows more resistance to further degradation.