The biodegradable polymers of poly(DL-lactide-co-glycolide) (PLGA) and poly(ethyleneglycol) (PEG) have come into use in the last few years for the control of the release of drugs. The incorporation of units of PEG produce an increased in nu iota nu o half life of the microparticles. The methoxylation of the PEG allows a control of the microparticles. The methoxylation of the PEG allows a control of the polymerization reaction by blocking one of the reactive terminals. An important aspect to be studied is the degradation of these polymers. The objective was centred on comparing the degradation that microspheres made of PLGA and PLGA-PEG (also with derivate methoxylated PEG). Degradation of the microspheres was studied at 37 degrees C and neutral pH. Samples were taken at different times to determine the size distributions, the mean diameter, the molecular weight and the structural composition of the polymer. The fastest decrease in Mw is observed in the case of PLGA microspheres. The PLGA-PEG microspheres show a significant size increase (from 24.6 to 90 mu m) due to a swelling process. Any appreciable changes in Tg values were observed during the assay (30 days).