Sorption equilibrium and diffusion coefficients of solvents in a polymer were determined by use of a magnetic suspension balance (MSB). The binary systems poly(vinyl acetate)-methanol and poly(vinyl acetate)-toluene at 20, 40 and 60degreesC were investigated. The solvent activity was calculated with the Flory-Huggins and UNIFAC-FV model and the diffusion coefficient with the free-volume theory. The measured values are compared with literature data and Flory-Huggins interaction parameters and free-volume parameters were adapted to the measured data. (C) 2003 Published by Elsevier B.V.