Vapor-phase infiltration, a postpolymerization modification process, has demonstrated the ability to create organic-inorganic hybrid membranes with excellent stability in organic solvents while maintaining critical membrane properties of high permeability and selectivity. However, the chemical reaction pathways that occur during VPI and their implications on the hybrid membrane stability are poorly understood. This paper combines in situ quartz crystal microbalance gravimetry (QCM) and ex situ chemical characterization with first-principles simulations at the atomic scale to study each processing step in the infiltration of polymer of intrinsic microporosity 1 (PIM-1) with trimethylaluminum (TMA) and its co-reaction with water vapor. Building upon results from in situ QCM experiments and SEM/EDX, which find TMA remains within PIM-1 even under long desorption times, density functional theory (DFT) simulations identify that an energetically stable coordination forms between the metal-organic precursor and PIM-1's nitrile functional group during the precursor exposure step of VPI. In the subsequent water vapor exposure step, the system undergoes a series of exothermic reactions to form the final hybrid membrane. DFT simulations indicate that these reaction pathways result in aluminum oxyhydroxide species consistent with ex situ XPS and FTIR characterization. Both NMR and DFT simulations suggest that the final aluminum structure is primarily 6-fold coordinated and that the aluminum is at least dimerized, if not further "polymerized". According to the simulations, coordination of the aluminum with at least one nitrile group from the PIM-1 appears to weaken significantly as the final inorganic structure emerges but remains present to enable the formation of the 6-fold coordination species. Water molecules are proposed to complete the coordination complex without further increasing the aluminum's oxidation state. This study provides new insights into the infiltration process and the chemical structure of the final hybrid membrane including support for the possible mechanism of solvent stability.