A comparison between mathematical formulations for describing the transport phenomena occurring within desiccant and enthalpy wheels including axial heat and mass diffusion and a simplified version without these effects has been performed. Simulation results using a two-dimensional formulation were compared to determine under which conditions should these terms in fact be considered. The results include the variation of rotational speed, sorbent mass fraction, Biot numbers, and the aspect ratio of the porous desiccant substrate. The results show that the aspect ratio, together with the Biot numbers, are relevant parameters in determining whether axial diffusion should be included. (c) 2005 Elsevier Ltd. All rights reserved.