H-1 chemical shift changes of a newly synthesized anionic surfactant, sodium decyl naphthalene sulfonate (SDNS), show that its critical micellar concentration (cmc) lies between 0.82 and 0.92 mM, which is in agreement with that measured by the surface tension method (0.8 mM). The difference in aromatic ring current effect on the naphthyl protons, the different changes in their spin-spin relaxation times upon micellization, and the appearance of cross-peaks between intermolecular protons in the aqueous micellar solution give information about the relative arrangement of the naphthyl rings in the SDNS micelles.