The synthesis of a block copolymer containing poly(aryl ether sulfone) (PAES) and poly(aryl ether ketone) (PEAK) is an interesting route combining in a single material thermal resistance (glass transition temperature, T-g >200 degrees C) and high environmental resistance. The synthesis involves the functionalization of the PAES polymer with anhydride chain ends and its further reaction with a PAEK polymer having one amine chain end. A true triblock copolymer is obtained. The block copolymer as well as all intermediates of the synthesis have been fully characterized, for example by nuclear magnetic resonance, gel permeation chromatography and viscometry. Preliminary properties of the copolymer are increased compared with the PAES polymers. Moreover, the copolymer exhibits a T-g above 200 degrees C together with a melting temperature lower than 400 degrees C. Copyright (C) 1996 Elsevier Science Ltd.