Block copolymers of polysulfone-poly(alkylene oxide)-poly(dimethylsiloxane) have been prepared by the addition of preformed alpha,omega-bis(hydrogensilyl) poly(dimethylsiloxane) oligomers to allyl end-capped poly(alkylene oxide)-polysulfone. The hydrosilylation reaction, catalyzed by platinum, was employed for incorporation of the siloxane chain into the copolymers in a 1 : 1 or 1 : 2 molar ratio of Si-H-terminated polydimethylsiloxane to allyl end-capped polysulfone. The products were characterized by IR, H-1-NMR, and gel permeation chromatography. The thermal stability was determined by thermogravimetric analysis. Differential scanning calorimetry was used to investigate microphase separation in the block copolymers.