Differential pervaporation measurements with alcohols in poly(butylene terephthalate) (PBT) in the range of its glass transition temperature were performed. The applied apparatus is based on a gas-chromatographic detection system. The concentration dependency of the diffusion coefficient prevents solving the diffusion equations analytically. Therefore, a method was developed using finite differences in space and time to obtain numerical permeant fluxes under the assumption of certain concentration dependencies D(c). The results of this procedure were compared with the measured permeant fluxes, and intrinsic diffusion coefficients D-0 and explicit functions D(c) were attained. In addition, the concentration profiles of the liquids in the membrane for all times of the pervaporation process could be displayed. The evaluation of the temperature dependency of D(c) shows maxima for the plastification effect of the alcohol molecules at a temperature near T-g.