Coadsorbed molecules in a ZSM-5 zeolite are directly shown to decrease or increase the diffusion rates of each other at high coverages. For an ethanol/methanol mixture, ethanol diffused faster and methanol slower than the corresponding pure material. For an acetone/methanol mixture, however, both molecules diffused slower than pure acetone, which is the slower diffusing molecule. Diffusion rates were directly measured at steady-state conditions using liquid feeds and isotopic labeling during transient permeation through a ZSM-5 zeolite membrane.