The interaction of diphenylamine-4-diazonium salt (DDS) with sodium dodecyl sulfate (SDS) in aqueous solution was investigated. The results show that in a 2.1-2.3 molar ratio of SDS/DDS, the solution viscosity increases suddenly to a thousand times the original and then drops rapidly as the ratio beyond the region. The dramatic increase in the viscosity was proposed to be due to aggregated micelles that form in solution due to the interaction of diphenylamine diazonium salt and micelles formed from SDS, and when further SDS was added drops rapidly, because the aggregated micelles were separated by the electrostatic repulsion force originated from the overfeeding of SDS. It is interesting that the viscosity of the solution is very susceptible to UV light, i.e., the increased viscosity decreases gradually when the solution was exposed under UV light because the irradiation decomposes the diazonium group. The influence of [SDS] and the ratio of SDS/DDS on the viscosity of the solution were also investigated. Copyright 1999 Academic Press.