Surfactant influences the ER response in two different ways. At low surfactant concentrations, it enhances the ER response by enhancing the particle polarizability. While at large concentrations, the response degrades due to the non-linear conductivity in the continuous phase. The yield stress is proportional to the electric field strength squared at small surfactant concentrations, but increases more slowly with field strength at large concentrations. In this paper, an interfacial polarization model is introduced to predict the ER behavior of surfactant-activated ER suspensions. Maxwell-Wagner model was modified by incorporating the effects of surfactant adsorption and field-induced alteration of the surfactant structure. The modified interfacial polarization model predicts well the qualitative behavior of the surfactant activated ER suspensions over all surfactant concentration ranges.