An aromatic polysulfone ether/polythioether sulfone copolymer having the formula: ##STR1## wherein each of R.sup.1 and R.sup.2 which may be the same or different, is hydrogen, halogen or a hydrocarbon group having from 1 to 8 carbon atoms, each of a and b which may be the same or different, is an integer of from 0 to 4, Ar is a bivalent aromatic residue selected from the group consisting of: ##STR2## wherein each of R.sup.3, R.sup.4 and R.sup.5 which may be the same or different, is hydrogen, or a hydrocarbon group having from 1 to 8 carbon atoms, each of c to e which may be the same of different, is an integer of from 0 to 4, Y is a single bond, --O--, --S--, --SO--, --SO.sub.2 --, ##STR3## wherein R is hydrogen or a hydrocarbon group having from 1 to 6 carbon atoms, and each of m and n is an integer indicating the degree of polymerization and m:n is from 1:20 to 20:1, said copolymer having an inherent viscosity .eta..sub.inh of from 0.1 to 1.5 as measured at 30.degree. C. with respect to its solution having a concentration of 0.5 g/dl in a solvent mixture of phenol/1,1,2,2-tetrachloroethane (3/2 by weight ratio).