Simulation of air separation in zeolites requires an accurate mass transfer model particularly under rapid cycling conditions. An earlier study [Todd et al., 2005. Adsorption 11, 427-432] examined the application of the dusty gas model to the simulation of the adsorption Of O-2/N-2 system in lithium-exchanged low silica-to-alumina ratio x-type zeolite (LiLSX) pellets. In that study we characterised Knudsen diffusion (C-K) and viscous flow (C-v) structural parameters, two of three parameters that constitute the viscous flow plus dusty gas model (VF + DGM) intrapellet flux equation for a macroporous pellet under adsorbing conditions. This paper quantifies the third structural parameter (C-m), related to molecular diffusion that arises when a multicomponent gas mixture is introduced into the pore network of a sorbent pellet. With these three parameters defined, the mass transfer behaviour for the O-2/N-2 system in our adsorbent is completely characterised (under the assumption of no surface diffusion). To experimentally characterise C-m, a series of breakthrough experiments were performed with a packed bed of LiLSX pellets. Six independent breakthrough runs over a range of conditions were performed with one of these runs used to determine C-m. The remaining five runs were used for validation. Our adsorption simulator previously described [Todd et al., 2003. Computers and Chemical Engineering 27, 883-899] was enhanced with the addition of a rigorous wall model and was used to perform all breakthrough simulations. A value of C-m = 0.122 was obtained giving a molecular diffusion tortuosity of 2.5. This is typical of tortuosities for porous adsorbent pellets. Experiments and simulations revealed operating conditions were very close to adiabatic during each experimental breakthrough run while a sensitivity analysis on the intrapellet structural coefficients revealed the sensitivity coefficients of C-m and C-K are similar in magnitude suggesting both mechanisms of diffusion are important. Viscous flow was several orders of magnitude smaller and hence is of minor importance in breakthrough simulations. Simulations also suggest the level of radial discretisation within a sorbent pellet is an important aspect of a discretised pellet model to consider when simulating a non-isothermal and non-isobaric breakthrough experiment. Relatively high pellet discretisations are needed to obtain physically meaningful structural coefficients. (c) 2005 Elsevier Ltd. All rights reserved.