Abstract In surfactant–oil–water phase behavior studies, especially those related to enhanced oil recovery, each of the oligomers of commercial polyethoxylated non-ionic surfactants partitions in a different pattern into the two or three phases in equilibrium, and a simple, fast, and accurate method is required to analyze all phases. Up to now, only oil-phase GC was relied upon to determine the partitioning without closing the surfactant inventory. Isocratic normal phase HPLC with a ternary mobile phase (n-heptane–chloroform–methanol 70–10–20) is shown to comply with these requirements to analyze the oligomer distribution in microemulsion, excess oil, and excess water phases of an optimum formulation system.