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We report the microstructure and properties of polyimide molecular composite films fabricated from a rigid rod-like polyimide [poly(pyromellitic dianhydride-p-phenylene diamine) (PMDA-PDA PI)] and a flexible coil-like poly(amide-imide) [poly(trimellitic anhydride chloride-4,4'-oxydianiline) (TMAC-ODA PAI)]. The molecular composite films were prepared by the thermal imidization of precursor molecular composite films consisting of soluble precursors of PMDA-PDA PI and TMAC-ODA PAI. The microstructure of the polyimide/poly(amide-imide) molecular composite films was characterized by infrared spectroscopy and X-ray diffraction. The thermal, optical, dielectric, and mechanical properties of the molecular composite films were examined using a thermogravimetric analyzer, differential scanning calorimeter, prism coupler, impedance analyzer, and universal testing machine. The results showed that the molecular composite films exhibited a single glass transition, indicating good compatibility between the two components. However, most of the properties showed a nonlinear trend with the compositions in the presence of a critical point at PMDA-ODA PI/TMAC-ODA PAI = 40/60 (wt/wt).