Novel copolymers of poly(ether sulfone) (PES) containing ring trifluoromethyl groups have been synthesized by starting from trifluoromethyl aryl sulfone monomers. Materials incorporating CF3 groUps Ortho to the ether bond have low molecular weights due to the instability of the CF3 group in the polymerization process. Higher-molecular-weight materials, involving CF3 groups ortho to the sulfone group, can be prepared : these materials show a number of modified properties when compared to PES, such as crystallinity and a lower uptake of water, although they are also somewhat less thermally stable.