The effect of processing on the structure and properties of polyethersulfone (PES) has been studied. Processing at normal processing temperature (315° – 335°C) has been carried out by kneading for different times and at various roller rotating speeds, and also by successive extrusion cycles. Changes in PES appearance, melt viscosity, molecular weight, and mechanical properties after processing were found. The thermal stability of PES during processing is different from that measured by thermal analysis. A phenomenon of thickening of melt during processing of PES was found by the present authors. It seems that this phenomenon was caused by prolonged shear, heat, and oxidation at the processing temperatures used.