Permeation of n-hexane and 2,2-dimethylbutane (DMB) through two tubular ZSM-5 zeolite membranes was studied as a function of temperature and isomer partial pressures. The maximum permeation selectivity is 650, obtained at high n-hexane pressures. Though the single-gas isomers permeate at similar rates, the presence of n-hexane decreases the DMB permeance dramatically, whereas n-hexane permeation is unaffected by DMB. The nonzeolite pores appear to be different in size or number in the two membranes, whose pores saturate at different n-hexane pressures. Membranes with smaller nonzeolite pores exhibit pore saturation. The dependence of permeance on partial pressure and temperature indicates that most of the DMB permeance is through nonzeolite pores, which can be blocked by preferential n-hexane adsorption. These pores are larger than zeolite pores but are apparently in the nanopore size range.