The interaction of sodium dodecyl sulfate(SDS) with cationic polyelectrolyte derived from poly(4 vinyl- pyridine styrene) hes been investigated. It was found that there has been a narrow molar ratio region of SDS/cation at which the viscosity of aqueous solution rises suddenly due to the aggregation of the SDS attached on cationic polymer

then falls sharply as the SDS/cation ratio is beyond the region because of the deaggregation. At a suitable condition a reversible gelation was observed. A suggestion for explaining the specific phenomena was proposed.