Organic solvent nanofiltration (OSN) and organic solvent reverse osmosis (OSRO) have the potential to revolutionize the chemical industry if used in a widespread manner. Many researchers focus on developing new materials which surpass the upper bound, however little research is focused on understanding fundamental transport mechanisms behind OSN and OSRO. This paper analyzes the validity of the solution diffusion model to describe solvent transport through glassy polymers. Flux decline in glassy polymers is often attributed membrane compaction, which is commonly used as evidence that the pore flow model describes flow through glassy polymers. However, this paper demonstrates how the cause of flux decline is thermodynamic in origin using the NELF model, and cause by a limit in concentration gradient in the membrane being reached. The validity of this hypothesis is verified with a system of PTMSP and ethanol. The role of solvent concentration in membranes is often understated in the development of new materials. Molecular interactions play a significant role in sorption as well as overall concentration. Celazole PBI membranes were tested in a wide variety of solvents, resulting in a non-monotonous sorption trend with lower alcohols. Lower alcohols can form competitive hydrogen bonds with Celazole, causing plasticization. This leads to a significant reduction in Celazole's otherwise strong chemical, thermal, and physical properties. This finding is supported numerically through an analysis of the isosteric heat of sorption as well as in situ FTIR. These findings highlight the importance of testing materials in realistic conditions in order to determine their separations properties.