Diffusion of dichloromethane in poly(lactide-co-glycolide) (PLGA), the rate-limiting step in the later stages of drying of microparticles formed in common encapsulation processes, was studied by the step-change sorption technique in a dynamic vapor sorption apparatus. Methods were developed to create films of polymer with the appropriate thicknesses for accurate diffusion determination over a wide range of solvent composition. Mutual diffusivities were measured at 5, 25, and 35 degrees C from 10 to 70 wt % solvent. Values range from 2 x 10-(10) m(2)/s at high solvent compositions to as low as 1 x 10(-13) m(2)/s at solvent compositions just above the glass transition of the mixture. Equilibrium sorption isotherms were measured in the same apparatus and agreed favorably with Flory-Huggins theory using a value of chi = 0.31. The glass transition temperatures of the system were measured over the range of 0-11 wt % solvent content by modulated differential scanning calorimetry. The composition dependence was fit to the Fox equation, which estimated values of the pure polymer and the solvent T-g to be 39.3 and -131 degrees C, respectively. These values, along with the diffusivity data, were used to deduce the free-volume parameters specific to PLGA. (C) 2009 Wiley Periodicals, Inc. J Appl Polym Sci 112: 1622-1629, 2009