Aggregation behavior of sodium salt of mono-n-dodecyl phosphoric acid ester i.e. a species of mono-alkyl phosphate (MAP) surfactants, in pure water was investigated as a function of surfactant concentration (C_S) by means of surface tension (γ), electroconductivity (measuring both specific (κ) and molar (Λ) conductivities), pH and UV-Vis spectra measurements. The Krafft temperature was determined as 40°C by observing the change of κ with raised temperature. Although the curve of γ vs. log C_S plot gave only one break which corresponds to critical micellization concentration(CMC) in common theory, different relations of κ vs. C_S, Λ vs. √C_S, or κ vs. log C_S, and pH vs. C_S resulted in several breaks suggesting the occurrence of various transitions of aggregates or micelles over the concentration range not only above, but also below CMC. The CMC determined by surface tension measurement was referred to as the first CMC (CMC₁). The critical concentrations of the transitions observed by various methods were termed as CAC_i (i=1,2,3) for three-step aggregate formation below CMC₁ and as CMC_j (j=1-5) for five-step transitions in micellar form. From the temperature study on CAC_i and CMC_j, the estimated standard Gibbs energy changes were estimated, which showed us that the primary driving force of aggregate/micelle formation at any step comes from the entropy term, since the energy term (enthalpy) in this case was found to be positive. This supports our interpretation that the aggregates/micelles are mainly composed of hydrolyzed (protonated) mono-dodecyl phosphate molecules so that they behave as if they were nonionic surfactant micelles. A light scattering measurement verified the existence of aggregates formed below CMC. The experiment of catalytic activity assay revealed that SMDP aggregates formed in the low concentration range (ten times lower than CMC₁) were found to act as catalysts for the hydrolysis reaction of p-nitrophenyl acetate (pNPA).