beta-Cyclodextrin (CD), which is too large to fit into MFI pores, was deposited in the intercrystalline defects of MFI zeolite membranes in an effort to decrease flow through the defects and increase separation selectivity. The permeation behavior of molecules that swell MFI crystals (SF(6), n-butane, i-butane) and those that do not (He, H(2)), plus the permeation of i-octane, which only diffuses through membrane defects, were used to show that CD lodged in the defects and inhibited adsorbate-induced expansion of MFI crystals. As a result, the defect sizes decreased less with adsorption of molecules that swell MFI crystals. The CD blocked a fraction of the defect cross-section, and thus decreased the fluxes of H(2), He, and i-octane through the defects. In contrast, the flux of molecules that swelled MFI crystals either increased (as much as a factor of seven for some membranes) or decreased, depending on the membrane and the amount of CD deposited. The fluxes increased because CD acted like a shim in the defects so they could not shrink as much, but the fluxes decreased at high CD loadings so that more defect cross-sectional area was blocked than had been decreased by swelling. These results provide direct evidence that defect sizes decrease when some molecules adsorb in MFI pores of MFI membranes. They also demonstrate the difficulty of characterizing MFI membranes because defect sizes change in the presence of many adsorbates. Most of the CD could be removed by calcination, but enough CD or its decomposition products remained that even after calcination, defects did not shrink as much due to adsorption as they had before CD deposition. (C) 2010 Elsevier B.V. All rights reserved.