This paper provides a review of the measurement, data reduction, and correlation of diffusivities using the free-volume theory for polymer-solvent systems, particularly in the range of very low solvent concentration. The experimental methods discussed are inverse gas chromatography, gravimetric sorption, and pressure decay. The free-volume model is described in terms of its potential and limitations for correlation and extrapolation of diffusion data. Data are given and analyzed for a number of systems including two of practical importance: solvents in poly(vinyl alcohol) and bisphenol-A in poly(vinyl chloride). (C) 2013 Elsevier B.V. All rights reserved.