Surface segregation in polymer blend systems between 3,3',4,4'-biphenyltetracarboxylic dianhydride/4,4'-diaminodiphenyl ether (s-BPDA/ODA) polyimide and block copolymer based on polysiloxane-block-polyimide (SPI) has been investigated. These polyimide blends, having various compositions of the SPI, were processed by a solution casting method. The glass substrate used in the film-casting process shows significant effect on the migration of surface segregated species to enrich the air-exposed surface, whereas the more polar s-BPDA/ODA tends to remain close to the polar glass substrate. X-ray photoelectron spectroscopy reveals that even at low SPI concentration, the siloxane moieties in the block copolymer tend to segregate into the air side surface. Contact angle measurement evidently indicates an enrichment of the hydrophobic siloxane fraction on the blend film surface. The average water contact angle of glass side surface is 77 degrees C whereas that of the air side is about 102 degrees C in every blend ratio. This behavior confirms the surface segregation phase separation in these polymer blends. Finally, the surface morphology observed by atomic force microscopy also suggests segregation type of phase separation in these blend systems.