A poly(phenylene sulfide) material containing predominantly para-phenylene sulfide structural units and having a melt viscosity of at least 5 poises (at 300.degree. C.) may be treated or reacted with an alkali metal sulfide and/or hydrosulfide in a polar aprotic solvent at a relatively low temperature in the range of 150.degree.-230.degree. C. so as to introduce an increased amount of reactive thiolate or thiol end groups to the polymer molecule and hence to enhance the reactivity of the polymer material. The thus modified and highly reactive poly(phenylene sulfide) material exhibits an improved compatibility with other polymers and is expected to be useful in preparation of polymer alloys of improved or modified physical and chemical properties.