The effect of pH on the oxidative stability in oil–water systems was determined at 40 °C for 18 days by monitoring headspace oxygen depletion and concentration of conjugated dienoic acids (CDAs). Two types of pH solutions were prepared: a buffer solution and a mixture of NaOH and HCl. When the pH of the mixture of NaOH and HCl increased from 1 to 13, the oxidative stability in oil decreased significantly in the order of pH 1 > 10 > 7 > 4 > 13. In case of the buffer solution, the oxidative stabilities of oils were in the order of pH 4 = 7 > 10. Depending on the pH and buffering capacities, oxidative stability of oils were greatly influenced.