Abstract: Tribological properties of sodium oleate,sodium laurate,sodium octanoate,dimer acid potassium and a water-soluble ammonium phosphorothioate salt as additives in water-glycol fire-resistant hydraulic fluids were investigated. The wear scar morphologies were investigated by scanning electron microscopy and MicroXAM Interferometric Surface Profiler. The topography and tribochemical species of the worn surfaces were analyzed by means of X-ray photoelectron spectroscope. The results show that the ammonium phosphorothioate salt had an excellent extreme pressure properties up to 1 117 N. The tribological results show that the ammonium phosphorothioate salt had better antifriction capability and the average friction coefficient was about 0.10 with the concentration of 1.0%. The XPS results show that the presence of Fe²⁺、Fe³⁺、PO₄^3-、and S^2- on wear scar and complex tribochemical reactions were involved during the course of friction.