ASP (Alkali/Surfactant/Polymer) flooding has been successfully applied in several field pilots in the Daqing oil field in China. Oil recovery has improved by about 20% OOIP over recovery by waterflooding. The interfacial tension (IFT) of ASP/crude oil is one of the most important factors that control the effectiveness of ASP flooding. The effects of alkali, surfactant, polymer and oil properties on IFT were previously studied. This paper addresses the effect of injection water quality on the IFT of ASP/crude oil. This paper analyzes the inorganic ion compositions and organic compounds of injected water in different pilots in Daqing. The effect of these compositions and the suspended particles in injection water on the IFT of ASP/oil were studied. A carboxylate surfactant was used in the study. All experiments were carried out at 45 degrees C. Results showed that the suspended particles had little influence on IFT, and that inorganic ion compositions had some influence to a certain extent but were not the main factor affecting IFT for the low salinity injection water used in ASP flooding in the Daqing oil field. The main factor increasing the IFT is certain organic compounds in the injection water. The paper points out the main types of organic compounds influencing IFT. Some methods to lower IFT by controlling water quality are proposed.