Densities of aqueous solutions containing n-octylamine, 0.0500 mol kg(-1) cetyltrimethylammonium bromide (CTAB), and sodium chloride/potassium chloride have been determined and used to derive the standard partial molar volumes of n-octylamine at 298.15 K. The effect of the additives (sodium chloride, potassium chloride, and n-octylamine) on the polarities of the micelles have also been investigated by static fluorescence spectra using pyrene as a probe at 298.15 K. It has been observed that the postmicellar transitions of sphere-to-rodlike, as indicated by the amplified changes in the standard partial molar volumes of n-octylamine, are affected strongly by the addition of an electrolyte. The effect of sodium chloride on the postmicellar transition of CTAB is much weaker than potassium chloride. Polarity of the interior of the mixed micelle and therefore the solubilized site of pyrene vary with molality of the additives. This is considered to be caused by the difference in chemical structure and hydrophobic nature of the mixed micelle. The results of fluorescence spectra reveal the possible microscopic causes for the effect of additives on the postmicellar transition of CTAB.