Membrane introduction mass spectrometry uses a semipermeable membrane to enhance the response of a mass spectrometer. The chemical composition of the membrane determines its permeation rates to various species. Careful selection of the membrane may make it possible to characterize more definitively close members of a chemical family. This article gives the results of an initial survey of the permeabilities, as determined by MIMS, of a variety of polymers to a family of chlorocarbons that are often the major components of subsurface remediation concerns.