In flexed-plate impact tests at room temperature, injection-molded disks of polyether sulphone could not be broken or penetrated by an impactor with an incident energy of 238 J. A central hole 1 mm in diameter was sufficient to embrittle a high proportion of the specimens subjected to the same impact. The failures could be segregated into three classes. A few subsidiary tests on thinned specimens with no hole suggest that in this particular molded form, and under the specific test conditions used, polyether sulphone has a tough-brittle transition within the thickness range of 1.4 to 2.0 mm.