A broad equivalent weight petroleum sulfonate (TRS 10-410), and a narrow equivalent weight sulfonate (sodium pentadecyl benzene sulfonate), at a concentration range of 0.001 to 0.6 wt. pet. were studied with regard to partitioning, interfacial tensions, and solubilizing properties with a number of linear alkanes. For all these properties studied, there exists a concentration where they underwent sudden changes in slope. Calculation based on solubilizing data indicated formation of swollen micelles. This observation is discussed in term of critical micelle concentration, equipartitioning and formation of swollen micelles.