Divalent metals cations present in injection water can significantly influence the performance of alkali-surfactant-polymer (ASP) flooding. These cations such as calcium and magnesium react with the added chemicals to form their insoluble salts as precipitations. In this paper, an in situ precipitation inhibitor known as sodium acrylate is used to overcome the precipitation problems prevalent with ASP flooding. Fluid-fluid compatibility tests were performed to examine the performance of the in situ precipitation inhibitor using hard brine having large quantity of divalent metal cations. The effect of the precipitation inhibitor on interfacial tension was also investigated using various inhibitor concentrations. The in situ precipitation inhibitor showed an excellent performance in preventing calcium and magnesium precipitations and the solutions remained clear for 45 days at 80 °C. Further, as the inhibitor concentration increased to an optimal value, an ultra-low interfacial tension of 0.04 mN/m could be achieved. The advantage of the in situ inhibitor is the use of hard brines without the need for softening the injection water. © 2013 The Author(s).