Four random, differently ended (-Cl, -NH2, -OH, and -COO-), polyethersulfone/polyetherethersulfone (PES/PEES) copolymers were studied to investigate the influence of chain ends on thermal and rheological behaviors. The number average molar mass (M-n approximate to 9500 g.mol(-1)) and the PES/PEES ratio (40/60) of all copolymers investigated were checked by H-1 NMR spectra. Thermal degradations were carried out in the scanning mode and initial decomposition temperatures (T-i) and activation energy values of degradation (E-a) were obtained. Glass transition temperature (T-a) was determined by differential scanning calorimetry and complex viscosity (eta*) by rheological measurements in isothermal heating conditions (T = 270 degrees C). All parameters determined were largely affected by copolymer chain ends and decreased according to the same order, -OH > -NH2 > -Cl > -COO-. The results were discussed and interpreted. POLYM. ENG. SCI., 49:14771483, 2009. (C) 2009 Society of Plastics Engineers