A TEOM is used to elucidate the adsorption/desorption characteristics of alkylation reactants on USY- and beta-zeolites. Equilibrium adsorption isotherms were obtained on USY- and beta-zeolites using n-butane, isobutane and propane as proxy reactant molecules (T = 303-398 K, adsorbate partial pressure 0-1.2 bar). Analysis of the transient adsorption/desorption profiles of these molecules from either a bed of the zeolite or pelletized particles of the crystals (with mean size < 1 mu m) demonstrate that diffusion in the secondary meso-/macroporous structure formed in the packing or the pellets controls the overall sorption rates. The experimental adsorption/desorption profiles from the pelletized zeolites were regressed with available mathematical models to obtain effective meso-/macropore diffusivities for reactant molecules, and nearly perfect fits of the experimental and the modeled profiles. Taking into account the dead volume in the system, a criterion for reliable measurements of either micropore or mesopore diffusivities by the TEOM technique is derived. (C) 2009 American Institute of Chemical Engineers AIChE J, 56: 1285-1296, 2010