The hydrophobic association behavior of p-propylbenzenesulfonate ions (n-PBS- and iso-PBS-) around poly (allylammonium) cation (PAAH+) was studied by absorption and H-1 nuclear magnetic resonance (NMR) spectroscopy. In the presence of PAAH+Cl-, the broadening (hypochromism) of absorption band of PBS- were observed. In addition, all H-1 NMR signals of PBS- exhibited up-field shift which resulted from the intermolecular ring current shift. These results indicate the hydrophobic association of negative PBS-around PAAH+. The hydrophobic association arises from the accumulation of counterions around the polyion, and is stabilized by the hydrophobic interaction between propyl groups and the stacking interaction between benzene rings. The association of iso-PBS- ions is rather weaker than that of n-PBS-, suggesting that the steric hindrance of isopropyl groups prevents an effective association of hydrophobic counterions. Furthermore, through viscosity and Cl- activity measurements, it was found that the binding of associated PBS-’s to PAAH+ causes the change of its surroundings to hydrophobic character and its conformation.