The effect of different fluid environments on the properties of poly(ether sulfone) has been studied. It has been found that, among the solvents studied,ortho-dichlorobenzene greatly affects the ductility of the polymer, producing a catastrophic embrittlement. This results even after short sorption times at 20°C. Among the possibilities of solvent-induced crystallization, antiplasticization, chemical attack, and surface cavitation, the last seems to be the most responsible for the observed embrittlement.