The time-lag technique is one of the workhorses for the study of gas permeation through dense polymers. Through the years, numerous experimental results have been accumulated based on this technique primarily for diffusion and permeability of gases, but also for sorption coefficients determination. Many refinements have been developed to the original set of equations in order to take into account more complex situations (e.g. non-constant diffusion coefficient, heterogeneous matrix, simultaneous chemical reaction within the polymer matrix...). Nevertheless, attempts to apply the time-lag technique to organic vapors are scarce and, in that case, experimental results have often shown significant discrepancies compared to other techniques. We describe in this work for the first time how the non-instantaneous upstream pressure step condition, imperative for time-lag framework build-up. can hardly be achieved when working with an organic vapor. This phenomenon can lead to major errors if a classical data treatment of the time-lag results is performed. A rational transfer analysis based on Laplace transform enables however the changing upstream boundary conditions to be taken into account. This strategy leads to a simple correction term which gives a better data consistency. The application of this strategy is illustrated by a series of experiments performed with dichloromethane, toluene and acetone vapor diffusion through a dense PDMS membrane.