Novel polymerizable and nonpolymerizable surfactants bearing an active ester group, p-10-undecenoyloxyphenyldimethylsulfonium methylsulfate (UPDS) and p-lauroyloxyphenyldimethylsulfonium methylsulfate (LAPDS), were synthesized. Their aqueous solutions were characterized in terms of the critical micelle concentration (CMC), the micellar aggregation number (N-s) at a concentration slightly above the CMC, and the area per molecule at surface saturation for adsorption at the water-air interface (a(m)) at 25 degrees C, mainly to evaluate the hydrophilicity of dimethylsulfonium methylsulfate as the head group. The CMC, N-s, and a(m) values were, respectively, (3.0-6.5) X 10(-3) M, 42, and 105 Angstrom(2) for UPDS and (1.5-2.2) X 10(-3) M, 88, and 60 Angstrom(2) for LAPDS. These values for LAPDS were in good agreement with those for n-alkyltrimethylammonium bromide with an alkyl chain length corresponding to 15.5 carbon atoms. Based on the results and the hydrophobicity of the tail group, it was concluded that the dimethylsulfonium methylsulfate group has a hydrophilicity comparable to that of a trimethylammonium bromide group. The terminal double bond in the tail group of UPDS was also found to disturb a molecular orientation in both the micellization and the adsorption at the water-air interface. UPDS and LAPDS were readily reacted with 2-methoxyethylamine in water at room temperature to give their corresponding amides in high yields, being indicative of the high reactivity of their ester group.