Employing impedance analysis of quartz resonators, we have monitored the complex viscoelastic compliance of a 1.67‐&mgr;m‐thick film of poly(isobutylene) during swelling in toluene vapor. The optical thickness was simultaneously measured by ellipsometry. The acoustic model used for analysis is discussed. We can quantitatively derive the shear modulus. The results show a decrease of the glass transition temperature of about 60 °C. We suggest that the method is of high value for the investigation of sorption and drying kinetics in the regime of low solvent content. ©1996 American Institute of Physics.