To produce an impact toughened grade of 'Victrex' Polyethersulphone by blending with (A-B)n type Polyethersulphone/Poly(dimethylsiloxane ) [P.E.S./P.D.M.S.] block copolymers. A number of novel (A-B)n type P.E.S./P.D.M.S. block copolymers of varying block molecular weights have been synthesised and characterised. These have been melt blended with pure 4800P grade P.E.S. to yield a series of 'impact modified' P.E.S./P.E.S.-co-P.D.M.S. blends. Standard Izod and Tensile test pieces have been injection moulded from these blends, and these test pieces used in a study of the physical, chemical and mechanical properties of the blends. The melt shear viscosity of P.E.S. has been found to be substantially reduced by the presence of P.E.S./P.D.M.S. copolymers. This is believed due, at least in part, to the migration of the P.O.M.S. bearing copolymer to the surface of the blend during processing. Improvements in sharp notch impact strength of up to 43% have been observed in P.E.S./P.E.S.-co-P.D.M.S. blends containing as little as 2.5% copolymerised P.O.M.S. These improvements have been achieved at the expense of more modest reductions (no more than 24%) in tensile strengths. Examination of impact fracture surfaces has revealed that the copolymers promote localised plastic deformation in sharply notched P.E.S., the copolymer particles themselves undergoing cavitation during crack propagation. It has also been shown, however, that the copolymers suppress the gross plastic deformation usually observed in unnotched or bluntly notched P.E.S. The additive particles promote crack initiation in such specimens, and although they also facilitate localised plastic deformation, the overall result is usually a decrease in P.E.S. impact strength. Significantly, blends containing P.E.S. and P.E.S./P.O.M.S. copolymers do not appear to perform any better under impact than simple physical blends of P.E.S. and linear P.D.M.S. This is believed to be due to migration of the P.D.M.S. domains to the surface of the P.E.S./P.D.M.S. copolymers. Once at the surface, they inhibit the formation of the desired adhesive bond between the P.E.S. matrix and the P.E.S. domains in the copolymer. The presence of the copolymers does not appear to have any significant effect on the mechanical performance of P.E.S. as tested after immersion in a range of organic and inorganic reagents at room temperature.