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Vapor phase infiltration (VPI) is a vapor processing technique that converts polymers into organic–inorganic hybrid materials with modified properties. VPI of polymer membranes stabilizes against dissolution and swelling in organic liquids, opening up new opportunities for use in organic solvent reverse osmosis (OSRO) separations. However, the precise chemical structure of the infiltrated inorganic components remains poorly understood, limiting the potential to fully exploit process–structure–property relations for materials design and slowing the development of new hybrid membranes. This study explores the structural characteristics contributing to the chemical stability of PIM-1/ZnOxHy hybrid membranes through advanced spectroscopic techniques to clarify the chemistry and inorganic cluster formation in these materials that lead to enhanced stability in solvents that otherwise swell or dissolve the pure polymer. X-ray photoelectron spectroscopy (XPS) indicates a predominantly zinc hydroxide chemistry with higher proportions of oxide forming only at increasing cycle counts. Extended X-ray absorption fine structure (EXAFS) spectroscopy provides new understanding of the first and second coordination shells. These results indicate that the size of the clusters increases with prolonged VPI precursor exposure and additional VPI cycles, leading to improvements in membrane solvent stability. These findings offer a new understanding for how the physicochemical structure of these hybrid membranes can be characterized and then used to design for a desired performance.