The concentration dependences of the Fourier transform infrared spectra of aqueous solutions of sodiumn-pentanesulfonate,n-hexanesulfonate,n-heptanesulfonate, andn-octanesulfonate were studied in concentration ranges encompassing the critical micellization concentrations (c.m.c). Changes in wavenumber, bandwidth, and intensity of infrared absorption bands were used to monitor the changes in molecular association with concentration. The premicellar aggregation below the c.m.c. may accompany the association of the alkyl chain, but not the counter-ion binding with the SO3−group. Above the c.m.c, the front location of the counter-ion against the SO3−group at the micelle surface is demonstra