Polyphenylene oxide (PPO) was compounded with bisphenol-S on Haake Rheometer and the torque during mixing was recorded. The dynamic properties of the PPO compounds were analyzed with dynamic mechanical analyzer and the phase structure was examined with transmission electron microscope and differential scanning calorimeter. The effect of the added bisphenol-S on the motion of the chain segments, the fluidity, stiffness, and heat resistance of the PPO were studied. At mixing temperature 280 A degrees C the Bisphenol-S melted and dissolved in PPO; as the compounds cooled and solidified after molding the bisphenol-S precipitated and recrystallized, presented as 40 nm particles in the PPO matrix. Bisphenol-S reduces the viscosity of PPO melts but does not deteriorate the stiffness and heat resistance of PPO materials. Bisphenol-S is more efficient than polystyrene to reducing the viscosity of PPO. At 5 parts of bisphenol-S, the torque decreased 35%.