The kinetics of desorption of 1,3-diisopropylbenzene and 1,3,5-triisopropylbenzene from 0.9-mu m NaY-zeolite crystals by the zero-length-column (ZLC) technique are reported. Both molecules were found to have nonlinear equilibrium adsorption properties because the observed K values are approximately 10(5). Accordingly, the zero-loading diffusivities were extracted from the data using the nonlinear asymptotic form of the ZLC model. The desorption curve of 1,3-diisopropylbenzene was normal, but that of 1,3,5-triisopropylbenzene contained a discontinuity reminiscent of a percolation threshold. This was attributed to catalytic reaction, and the reported diffusivity for this material is hypothesized to be that of the codiffusion of diisopropylbenzene isomers with propylene. The reported activation energies are accordingly very similar for both species (approximate to 66 and 77 kJ/mol, respectively); the activation energy for 1,3-diisopropylbenzene is intrinsic, whereas that for 1,3,5-triisopropylbenzene is a pseudo activation energy.