The liquid-liquid equilibrium interfacial tension of binary systems of saturated butan-1-ol and water has been studied over the aqueous surfactant (sodium dodecyl sulfate) concentration range of 0 to 3.48 x 10(-4) mol(.)L(-1) (0 to 100 ppm), the pH range of 4 to 9, and the temperature range of 20 to 50 degreesC. The measurements were made by the drop-weight method. No previous data were found in the literature for the effect of surfactant and pH. The experimental data show a linear decrease with increasing surfactant concentration and temperature; however, the pH reduces the interfacial tension nonlinearly. The interfacial pressure was also obtained from data. The experimental values were correlated by empirical equations.