Three block copolymer systems where one block was that of an engineering thermoplastic were examined. The first was that of polysulfone-polyarylester block copolymers where glassy-crystalline block copolymers were synthesized and characterized. The morphology of the block copolymers could be controlled by varying the chemical microstructure of the polyester segment. Solvent resistance was shown to improve as the level of crystallinity was increased. Segmented block copolymers of an amorphous polyarylester and polydimethylsiloxane were also investigated. Both the random-block and perfectly alternating synthetic routes were used and it was found that the perfectly alternating technique produced a more regular morphology; than the random-block technique. A study was also carried out on the incorporation of small amounts of polysulfone-polydimethylsiloxane block copolymers blended into homopolysulfone. Improved fracture toughness was observed without a significant loss of stiffness. The morphology of block copolymer particles dispersed in the polysulfone matrix was directly observed through transmission electron microscopy.