We report on desorption measurements on polymeric thin films coated onto quartz crystal resonators. Due to the high sensitivity of quartz crystal microbalances, the experiments can be performed on very thin films : which have small diffusion time constants even in the glassy state. When drying is performed slowly enough, diffusion equilibrium can be maintained through the whole process of desorption, including the glassy domain. From these quasi-stationary pressure ramps, we derived the solvent chemical potential as a function of polymer volume fraction mu(phi) The results fit well to a model recently proposed by Leibler and Sekimoto.(1) In addition, we have derived the mutual diffusion coefficient D(phi) from pressure step experiments. We observe a strong decrease of D(phi) for high polymer concentrations typical of hypodiffusive systems like polymers. We investigated the drying of an industrial varnish that is a blend of 2 copolymers as well as the drying of its components separately. Both the solvent chemical potential mu(phi) and the mutual diffusion coefficient D(phi) of the blend interpolate between the respective quantities of the components.