Time lag permeation experiments are a well-known method for the determination of permeability and diffusion coefficients of gases in polymer films. In these experiments pressurized gas is brought in contact with one side of the film and the amount of gas evolving from the opposite, low pressure, side of the film is measured as a function of time. The permeability coefficient is obtained from the steady-state permeation flux and the diffusion coefficient is obtained from the permeation flux versus time behavior through an extrapolation method. The time required to reach steady-state permeation is of the order of several hours or less for most polymers, but can be of the order of weeks or even months for films prepared from low-permeability barrier polymers. A novel method is described in this paper for the early determination of the permeability coefficient, which was revealed to yield estimates at 1/10th the time it normally takes with the conventional time lag methods. This method requires (i) short-time approximation of the diffusivity coefficient, and (ii) accurate monitoring of transient gas flux in and out of the tested membrane. The diffusivity coefficient was determined based on a known, but little used work of Rogers et al. [1]. To measure the gas flux entering the membrane, a new design of the upstream side of a constant volume system was developed. The applicability of the shortcut method is demonstrated in permeation tests with nitrogen in a PPO membrane. The results show good compliance with the classical time lag and steady state methods. In the case of imprecise pressure measurements, improved accuracy of results are observed and expected when operating at higher pressures. (C) 2012 Elsevier B.V. All rights reserved.