We measured by the spinning-drop and occasionally by the sessile-drop method interfacial tensions against decane and hexane of aqueous preparations containing sodium 4-(1′-heptylnonyl)-benzenesulfonate (SHBS), sodium dodecylsulfate (SDS), and NaCl. We determined the phase behavior of these surfactants in water/NaCl mixtures by spectroturbidimetry, visual observations, polarizing microscopy, ultracentrifugation, 13C NMR spectroscopy, conductimetry, and differential scanning calorimetry. At 25°C and below 1 wt% NaCl in water, SHBS is less than 0.06 wt% soluble and forms micelles. SDS can solubilize into mixed micelles up to 0.35 ± 0.07 mole of SHBS per mole of SDS. SHBS can solubilize 0.25 ± 0.08 mole of SDS per mole of SHBS into mixed lamellar liquid crystals. The solubilized SDS causes the melting transition centered at −70°C of the hydrocarbon chains of SHBS liquid crystals to broaden. When only micelles of pure SDS or SDS mixed with SHBS are present, the interfacial tensions against hydrocarbons are higher than 1 mN/m. Lower tensions occur when liquid crystals consisting of either pure SHBS or SHBS mixed with SDS are present in the aqueous phase. Ultralow—less than 10 −2 mN/m—tensions, which are important for enhancing oil recovery, require the presence of dispersed liquid crystals of composition that “suits” the hydrocarbon. This composition occurs at an optimal combination of salinity, SDS-to-SHBS weight ratio, and total surfactant concentration. We have concluded that ultralow tensions arise not from dissolved surfactant but from these dispersed microcrystallites which by taking up hydrocarbon form a surfactant-rich third phase at the hydrocarbon/aqueous interfacial region. On the basis of this mechanism, we can suggest an explanation of the pronounced tension-concentration minima observed with multicomponent petroleum sulfonate commercial surfactants, such as Witco's TRS 10–80, whose behavior can be well-mimicked by our SDS/SHBS mixture.