The effect of temperature on three commercial surfactants (TRS 1080, Petrostep 450, and Petrostep 465) was studied. The surfactants were mixed in different proportions to vary the equivalent alkane number. The solutions contained 2g/L of surfactant mixture. Interfacial tensions were measured against mineral oil and n-dodecane. Sodium Chloride concentration was varied from 0.00 to 20 g/L. The experiments were conducted in the temperature range of 25 ℃ to 175 ℃, using the modified spinning drop tension meter. The interfacial tension for surfactant mixtures decreased above about 150 ℃. A minimum interfacial tension of about 10–3 dyne/cm was observed with a salt concentration of 10 g/L at a temperature of 175 ℃. The effect of various electrolytes (sodium–chloride, sodium sulphate, sodium carbonate, and sodium phosphate) was investigated as well. Of these, sodium chloride gave the lowest interfacial tension at all employed temperatures. Mixtures of anionic surfactant solutions have been aged for periods up to a year at room temperature. Interfacial tensions of these aged mixtures were higher than the fresh solutions. Since these solutions were not protected from air, the results are probably a result of slow decomposition of surfactants in the presence of oxygen.