The liquid-liquid phase separation diagrams for dilute aqueous NaCl solutions of dimethyloleylamine oxide have been obtained at temperatures between 5 and 85°C for pH less than 8 at different NaCl concentrations. The liquid-liquid phase separation is induced between pH 3.85 and 4.9 at temperatures less than 30°C, when the NaCl concentration is 0.05 M and the surfactant concentration is 0.30 × 10−2 g cm−3, and it occurs over wider regions of pH and temperature with increasing NaCl concentration. The critical value of lower consolute NaCl concentration, below which a homogeneous solution is formed, is 0.048 M for pH 4.6 at 25°C.The consolute phase boundary is also given as a function of surfactant concentration. On the temperature-surfactant concentration phase diagrams, an upper critical solution temperature (UCST) is observed for both 0.05 and 0.10 M NaCl solutions, and it decreases with decreasing pH. Similarly, a lower critical solution temperature (LCST) observed for 0.10 M NaCl solutions decreases with a decrease in pH. Since the LCST would be lower than the UCST at a given pH, the phase separation profile should be a “closed loop.” The pH dependence of liquid-liquid phase separation is discussed in relation to the hydration of rodlike micelles and the hydrogen bonding between amine oxide molecules and N-hydroxyammonium ions in a micelle.