The performance of barriers - widely used in packaging materials to prevent undesired environmental substances from contaminating the product - can be evaluated using, for example, steady-state permeation fluxes. While this method is simpler than the standard lag-time method it does, in contrast to the latter, not provide diffusion coefficients. Furthermore, permeation flux measurements depend on test conditions (vapor pressure, barrier thickness); this limits the usefulness of the flux method because comparison between films of different type and/or thickness becomes difficult. This work attempts to overcome such limitations for the case of a single barrier layer. ""Apparent"" diffusion coefficients are extracted from (transient) fluxes; the time of initial exposure need not be known. Additionally, the applied approximations allow steady-state migration for different conditions to be captured by a ""master line"".