Monoalkyl phosphate (MAP) salts are a kind of bivalent anionic surfactants. The difference of properties between half-neutralized monosalt and completely neutralized disalt is very interesting. In this study, the aggregation behavior of monopotassium monododecyl phosphate (MAP-12K) in aqueous solution with an increase in concentration was investigated by surface tension (gamma), elemental analysis, gas chromatography, differential scanning calorimetry, steady-state fluorescence, and negative strained transmission electron microscopy techniques. MAP-12K aqueous solution showed some characteristics: (I) Vesicle aggregates were formed at very dilute concentration (1.2 mM). (II) The precipitate of a highly hydrophobic dirner of MAP, which was quaternary neutralized by potassium, was generated only in a certain dilute concentration region (2.7-200 mM) around the critical micelle concentration (cmc = 20 mM). (III) Vesicles spontaneously translate into micelles at the cmc. (IV) In the higher concentration above 200 mM, the solution becomes homogeneous micellar solution. All of these uncommon characteristics are thought to be caused by the generation of the dimer, which is much more hydrophobic than dissolved MAP derivatives, in the complicated chemical equilibria based on the weakly acidic character of MAP. MAP-12K aqueous solution behaves as if it is a binary mixed surfactant solution of hydrophobic dialkyl surfactant and hydrophilic monoalkyl surfactant in spite of a single component solution.