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Molecular dynamics (MD) simulations of more than 40 model systems of dry and hydrated poly(pyromellitimide-1,4-diphenyl ether) (PMDA-ODA) polyimide, also known as Kapton, were undertaken in order to study the specific interactions between water and a glassy polymer matrix. Dry amorphous Kapton models were generated using a hybrid pivot Monte Carlo-MD single-chain sampling technique and were found to reproduce many features found experimentally. Water was subsequently introduced into the polyimide systems with weight percentages ranging from 1.4 to 10 wt % at 373 K. At the lower concentrations, water is found to occupy the pre-existing void space in the polymer matrix. On the other hand, there are significant changes in the volumetric, energetic, and conformational properties of the chains for water contents above ∼3%. The static and dynamic properties of the hydrogen bonds have been analyzed. In all cases under study, water is preferentially hydrogen-bonded to two sites, thus forming bridges. These water bridges, whose occurrence depends on the weight percentage of the penetrant, can link either two polymer sites, one polymer site and another water, or two other waters. Water clusters have been analyzed and found to be in agreement with findings from NMR, dielectric, and infrared attenuated total reflection (ATR) measurements. Their geometries are more chainlike and at high water contents approach a cocontinuous network. Such a characterization at the molecular level calls into question the commonly used interpretation of solubility curves based on the Zimm−Lundberg theory.