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Determining a stable surfactant mixture solution for surfactant flooding is crucial to assess the performance of the surfactant in improving the recovery factor. To evaluate the stability of the surfactant mixture solution, cosolvent is introduced to observe how the mixed solution system behaves. This study is focused on determining the optimal concentration of the cosolvent for a pre-defined surfactant mixture with the aim of comparing the effects on the surfactant mixture system behavior from two different cosolvents (IPA & EGBE). This study utilizes zwitterionic and anionic surfactants in a surfactant mixture, with the formula obtained from previous experiments. MA-AO14 and AR-M are specifically chosen as the zwitterionic and anionic surfactants, respectively. The selection of cosolvents includes commonly used laboratory testing cosolvents (IPA) and those with flash points similar to Indonesian oil reservoir temperatures (EGBE). The study involves conducting various tests such as aqueous stability, phase behavior, and interfacial tension measurements using a spinning drop tensiometer. The study reveals that the addition of IPA as a cosolvent has a more favorable impact compared to EGBE. IPA enhances the performance of the surfactant mixture solution by reducing the average IFT, improving solubility, and lowering viscosity. The optimal concentrations of IPA are found to be 0.25% w/w and 0.40% w/w of IPA, resulting in IFT reductions to 2.30×10?3 mN/m and 1.72×10?3 mN/m, respectively. Initially, the surfactant mixture solution without any cosolvent had an IFT of 2.91×10?3 mN/m. The results indicate that the addition of IPA as a cosolvent plays a crucial role in stabilizing the aqueous stability of the surfactant mixture and reducing the IFT between the oil and surfactant solution at a laboratory scale. This study highlights the potential of the surfactant mixture formula as a viable option for optimizing chemical EOR processes.