The dependence of morphology of the poly(imide siloxane)s (PISs) on the solubility parameter of unmodified polyimides and the molecular weight and content of β£,-bis(3-aminopropyl) polydimethylsiloxane (APPS) has been studied. The effect of the morphology on the mechanical properties is also under investigation. The domain formation in the PISs with the APPS molecular weight M n Ο 507 g/mol is not found until the mol ratio of APPS/PIS Υ 0.5% in the pyromellitic dianhydride/p-phenylene diamine (PMDA/p-PDA)-based PISs, and at a mol ratio Υ 2.7% in the 3,3Π,4,4Π-benzophenone tetracarboxylic dianhydride/2,2Π-bis[4-(3-aminophenoxy) phenyl] sulfone (BTDA/m-BAPS)-based PISs. As the APPS M n Ο 715 g/mol, the critical APPS concentrations of the domain formation in both types of PISs are equal to 0.1 and 1.1%, respectively. The critical concentration is equal to 0.6% in the BTDA/m-BAPS-based PIS film with the APPS M n Ο 996 g/mol. The isolated siloxane-rich phase in the BTDA/m-BAPS-based PISs becomes a continuous phase as the mol ratio of APPS/PIS Υ 7.7, 10.0, and 16.6% as the APPS M n Ο 996, 715, and 507 g/mol, respectively. Dynamic Mechanical Analysis (DMA) shows two T g s in the PIS films having phase separation: one at Οͺ118 Ο³ -115Β°C, being the siloxane-rich phase, the other at 181-244Β°C, being the aromatic imide-rich phase. The SEM micrographs show a significant deformation on the fractured surfaces of the BTDA/m-BAPS-based PIS films with a continuous siloxane-rich phase. This phenomenon of plastic deformation is also observed in the tensile tests at Οͺ118Β°C and at room temperature. The highest elongation in the PIS films is found at the critical siloxane content of the continuous siloxane-rich phase formation.