In this paper, a powerful and simple method to evaluate quantitatively the influence of the non-ideal contribution of the hydrogen permeation under diffusion control is provided. For this purpose, a Non-Ideality Index (a) is defined as the ratio of the non-ideal permeability term and the overall permeability, this leading to a direct quantification of the non-ideal effect in membrane. The used methodology is described in detail by using experimental data of a real Pd-membrane. From these data, the parameters that characterise permeability as a function of temperature and pressure - i.e., intrinsic permeability (b) and non-ideal factor (a) - are evaluated, highlighting their higher suitability in describing the membrane behaviour with respect to the usual empirical parameters used by the majority of researchers (i.e., pressure exponent and corresponding permeability). It is underlined that the proposed approach is particularly effective for modelling membrane contactors and reactors, where the empirical Sieverts-type law cannot be used because the large difference of pressure and temperature could lead the membrane behaviour to dramatically change even at different locations of the same device. Copyright (C) 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.