Methods that use n-hexane (n-hexane permporosimetry and n-hexane/2,2-dimethylybutane (DMB) separation) are shown to not be effective for characterizing MFI zeolite membranes because n-hexane adsorption swells MFI crystals and shrinks the size of nonzeolitic pores. Measurements on a membrane in which 30% of its helium flux at 300 K was through nonzeolitic pores demonstrate that benzene permporosimetry and isooctane vapor permeation as a function of feed activity provide better characterizations. Isooctane condensed in nonzeolitic pores at high activities, and this was used to estimate the sizes of those pores. The average nonzeolitic pore size in this membrane decreased from approximately 3.0 to 1.5 nm as the temperature increased from 300 to 348 K, apparently due to thermal expansion of MFI crystals. Benzene permporosimetry yielded dramatically different results from n-hexane permporosimetry because benzene does not swell the MFI crystals significantly. Single-component pervaporation fluxes as a function of molecular kinetic diameter verified the results from benzene permporosimetry. Larger molecules had higher fluxes than n-hexane because they diffused through nonzeolitic pores that were shrunk by n-hexane adsorption. Nonzeolitic pores were estimated to account for only 0.5% of the membrane permeation area, but 30% of the helium flux, because these pores were significantly larger than MFI pores.