The binding isotherm of sodium dodecyl sulfate (SDS) on a hyperbranched polyethyleneimine (PEI) was determined by an equilibrium dialysis method. Dynamic light scattering, electrophoretic mobility, and coagulation kinetics measurements were also performed in order to monitor the changes in the charged nature and size of PEI/SDS complexes. The experimental binding isotherm shows that the SDS interacts with PEI in two different ways. In a first binding process, the dodecyl sulfate ions bind in monomer form to the protonated amine groups, which is accompanied by an increase of the pH. A quantitative model is presented to describe the relation between the surfactant binding and the pH change. Above a critical amount of the bound surfactant, the PEI/SDS complex molecules collapse and precipitate. After the collapse of the polyelectrolyte/surfactant molecules, the SDS adsorbs on the surface layer of the collapsed particles (causing a charge reversal). This means that the interaction of the SDS with PEI can be divided into different characteristic SDS concentration ranges. At low surfactant concentrations, the system is a thermodynamically stable solution of the polymer/surfactant complex molecules. Above this critical concentration, the system is an unstable colloid dispersion of the complex particles. At even higher surfactant concentrations, the system may be a kinetically stable dispersion of the PEI/SDS particles, depending on the method of preparation. It can be concluded that the observed mechanism of PEI-SDS interaction is different from the general characteristics of the oppositely charged linear polyelectrolytes and surfactants, where the precipitated complex dissolves in the excess surfactant due to a collective (micelle-like) polymer-surfactant interaction.