The partitioning of a micelle-forming octyl phenol ethoxylate surfactant (Triton X-100) between water and sunflower oil was investigated and quantified as partition coefficient P, i.e. the ratio of total surfactant concentrations (molecules dissolved and in micelles) in oil and water. Values of P were determined over 5 decades of the concentration in the aqueous phase. Below the critical micelle concentration (cmc), P = 2.0 was found to be independent of concentration over a range of more than two decades. When adding surfactant to an oil-water two-phase system at or beyond the cmc, all extra surfactant accumulates in the water phase, and the value of P reduces accordingly. Arguments are given why emulsification leads to an O/W emulsion while Bancroft’s rule as based on the molecular preference of the surfactant for oil (P = 2.0), would suggest the opposite. Bancroft’s rule should be taken in its original form, taken into account all oil in both phases.