The water uptake of poly(DL-lactide) (PLA) and its copolymer with glycolic acid (PLGA 50: 50) has been studied using a quartz crystal microbalance (QCM). The polymers were swollen in humid atmospheres at 20 degreesC in order to probe both the kinetics of adsorption and the equilibrium amount of water taken up at low concentrations. The relative humidity of the air was controlled using saturated salt solutions. By equating the chemical potentials of the water vapor and the polymer/water mixture at equilibrium, it was possible to extract the Flory-Huggins interaction parameter chi from a simple binary mixing theory. Studies were also performed on the formulations of these materials with a model polypeptide active ingredient. The kinetics of swelling were described using a model based on the Thomas-Windle model of penetrant diffusion in glassy polymers.