The phase structure of polysulfone/poly(tetramethylene oxide) multiblock copolymers (PSU/PTMO BCP) strongly depends on the molecular weight of the coupled blocks. Non-phase separated structures are obtained using low molecular PTMO blocks. These block copolymers show a single T-g that follows the miscibility rule by Fox. Block copolymers with longer PTMO blocks have a phase separated morphology that is characterized by the coexistence of an amorphous PSU phase, an amorphous PTMO phase as well as crystallized PTMO domains. The influence of the block copolymer composition in blends with high molecular PSU was investigated. Blends of PSU and PSU/PTMO block copolymers are miscible if the molecular weight of the PSU segment exceeds the entanglement molecular weight twice. Otherwise, partial phase separation into a PSU-rich and BCP-rich phase is observed in the blend.