Viscosity measurements have been performed on 0.3 M sodium dodecyl sulfate (SDS) micellar solutions containing different ammonium salts, NH4X (X = OH, Cl, Br, I, NO3, or SCN) at different temperatures (25 to 40 degrees C). The viscosity decreased, increased, and again decreased as the salt concentration was increased. This behavior was explained in the light of micellar growth/breakdown, the nature of X, and intermicellar interactions. It was also found that, depending on colon species of the added salt, the viscosities of SDS/NH4X micellar solutions differ considerably, indicating the effect of hydrophobic interaction on micelle size or viscosity.