Adsorption of i-butane, p-xylene, and benzene shrinks the size of MFI crystals in polycrystalline MFI membranes at room temperature. The crystal shrinkage, which is over different loading ranges for the three molecules, increases the sizes of the defects (gaps between the crystals). Two membranes, one with 90% of its helium flux through defects, and one with 9% were studied in detail. For the membrane with 90% of its helium flow through defects, the flux through the defects increased by as much as 44%, depending on the permeating molecule. The defect sizes increased for p-xylene loadings below 4 molec./u.c. and for i-butane loading between 3 and 8 molec./u.c. At higher loadings (7 molec./u.c. for p-xylene and 9 molec./u.c. for i-butane) p-xylene and i-butane expanded MFI crystals and shrank the defect sizes so that the helium flow through defects decreased by as much as two orders of magnitude. Benzene did not significantly change the flux through defects at loadings below 8 molec./u.c. Near a loading of 8 benzene molec./u.c. the flux through the defects increased by approximately 18% for one membrane. (C) 2009 Elsevier B.V. All rights reserved.