This work presents a methodology for characterising solute transport through pervaporation membranes or, more generally, through dense membranes, in the whole transient regime. A real-time characterisation of transport through dense membrane is obtained by using on-line mass spectrometry (MS) monitoring, which allows to acquire the concentration of solutes in the permeate compartment with time intervals of 2 s (and shorter if required). Time-dependent diffusion coefficients, D(t), were calculated for the whole operation period, including the initial transient period. Based on these values it is possible to infer about the relevance of solute-membrane interactions and rearrangement of the membrane structure due to the presence of permeant solutes. Finally, based on the information acquired, a mathematical model was developed in order to obtain solute concentration profiles inside the membrane and their evolvement along time (C) 2015 Elsevier B.V. All rights reserved.