A series of polyimidesiloxane (SIM) copolymers, based on α,ω-aminopropylpoly- (dimethylsiloxane) (PDMS), 2,2-bis(4-[4-aminophenoxyl]phenyl)propane and 4,4′- oxydiphthalicanhydride was synthesized in our laboratories. The effects of siloxane segment length (dimethylsiloxane repeat units=1, 2, 3, 4, 5, and 9) and PDMS bulk content (1–30 wt %) on the surface composition and free surface morphology of the copolymers were investigated. Experimentally, angle-dependent electron spectroscopy for chemical analysis (ESCA) was used to acquire composition information of the copolymer surface at various sampling depths. Time-of-flight secondary ion mass spectrometry (TOF-SIMS) was used to describe ion formation mechanism of bulk SIM copolymers with variation of bulk composition or structure. In all cases, PDMS was segregated to the topmost 100 Å of the air (free) surface of ∼75 μm thick film, even with short siloxane segments and low PDMS contents. Angle-dependent ESCA measurements show that the dominant factor in defining the surface composition of these copolymers is not bulk composition, but in fact siloxane segment length. With the same PDMS content, a longer siloxane segment gives a surface richer in siloxane. For a given siloxane segment length, varying PDMS content does not significantly change the surface composition. TOF-SIMS measurements of bulk copolymers simulated by solvent-cast thin films show that a longer siloxane segment gives higher relative ion yields of fragment ions from siloxane versus imide segments even with the same 10 wt % PDMS content in the copolymers. For a given average siloxane segment length of 9 dimethylsiloxane repeat units, a higher PDMS content in the copolymers yields higher relative yields of fragment ions formed from siloxane versus imide segments. © 1998 American - - Vacuum Society.