The main result of this work is to provide a simple, correct treatment of MIMS data to extract the physically meaningful parameters D and K. Interestingly, the general feature that D from short time analysis lies significantly below that from long time analysis suggests a weakly non-linear diffusion process in the systems examined, i.e. that D increases weakly with concentration under the conditions used here. Indeed from previous experiments, and from free-volume theory, D typically increases with concentration for low molar mass organic compounds in polymeric membranes [16,17]. We are currently exploring whether this feature can be quantitatively analyzed from MIMS data. Because mass spectrometers are highly selective it is possible to simultaneously study multiple analytes with subtle structural differences. Also, since selective membranes can largely or completely reject all but a narrow class of components in a sample, (e.g. designed to reject hydrophilic species, ionic species, particulates) MIMS can be used to directly analyze "native" or "dirty" samples. This makes possible advances for several important applications: 1) the reliable detection of trace, target contaminants in "native" samples, 2) the direct investigation of analyte permeation from multi-component mixtures, 3) the rapid, accurate, comprehensive study of the mass transfer properties of new membrane materials.