A comprehensive study of the mass transport of two volatile organic compounds (VOCs) heptane and toluene and their binary mixtures through dense semicrystalline low density polyethylene membranes (LDPE) at 25 degrees C is presented. The use of the widest possible concentration range (VOCs activity of 0.001-1 at given temperature) and a set of four different experimental techniques enabled to reveal differences between permeation and sorption of studied vapours and to clarify the effect of feed and instantaneous VOCs concentration inside the LDPE membrane. The differential flow permeation method and fixed-volume pressure increase method were used for pure and binary vapour permeation while sorption measurements (from liquid and vapour phases) were performed gravimetrically and by the Vapour Phase Calibration technique (VPC). This work is the first and unique comparative study on a so wide range of vapour activities and a so vast number of different techniques, giving important insight into the transport phenomena themselves and on how results are affected by the specific technique used. Results are demonstrated and discussed in detail on the basis of a pair of penetrants with different physico-chemical properties and molecular shape (toluene vs. heptane). Information about differences between concentration dependent permeation and sorption, coupling effect, molecular clustering and polymer-solvent interactions is reported as well. The establishment of specific concentration profiles inside the LDPE membrane during permeation and sorption experiments evoke time-dependent structure changes of the LDPE matrix and such chain reorganisations consequently influence the transport parameters and they are also related to a certain form of high concentration-evoked molecular aggregation (clustering). X-Ray diffraction measurements excluded that the observed phenomena could be due to significant irreversible changes in the crystallinity of the material. Toluene and heptane favourably influence one another's transport and consequently both sorption and permeation experiments reveal a distinct coupling effect. Permeation experiments show a gradual increase of the integral diffusion with increasing vapour activity whereas VPC and gravimetric sorption analysis find a maximum around activity a=0.7. This difference is due to the deviation from Fickian behaviour at high activity and to fundamental differences in the measurement principle. (C) 2009 Elsevier B.V. All rights reserved.