Small-angle neutron scattering (SANS) was measured for aqueous solutions of amphiphiles with organic additives or counterions such as 2-indenecarboxylic acid, cinnamic acid, and salicylic acid or their ions. The effect of organic species on the supramolecular assembly structure was investigated. Equimolar mixtures of dodecyldimethylamine oxides and aromatic carboxylic acids constructed vesicles. The vesicular bilayer thicknesses were 29-30 Angstrom, independent of organic species. On the other hand, hexadecyltrimethylammoniums with aromatic carboxylate counterions always formed rodlike micelles with axial ratios 8-10. Aqueous solutions of tetradecyltrimethylammonium salicylate displayed a strong interparticle interference effect on SANS profiles at concentrations above 0.8 x 10(-2) g cm(-3), although such an effect disappeared with addition of more than 5/mM sodium salicylate.