Light scattering studies show that in aqueous solution poly(ethylene imine) (PEI) exists largely in the form of individual macromolecules plus a small fraction of aggregates. The aggregates make a large contribution to the scattering signal but only a very small contribution to the solution viscosity. Addition of sodium dodecyl sulfate (SDS) to the solution has a number of interesting consequences. Microcalorimetry experiments show that well below the critical micelle concentration of SDS, individual SDS molecules add to the PEI through an exothermic process. At higher SDS concentrations, there is a noncooperative adsorption, which is endothermic in nature, of SDS micelles onto the polymer chains. The surfactant-polymer complex likely contains several polymer molecules. These solutions are characterized by a higher specific conductivity than can be explained by the sum of the conductivities of all. the individual ions in solution, even if the Na+ and DS- ions were free in solution and not bound to the polymer. Pulsed-gradient NMR measurements were carried out to examine the Na+ and DS- ion mobility in the solutions. These measurements showed that surfactant binding to the polymer released sodium ions from the SDS micelles. The increase in pH showed that this binding also releases a small amount of OH-into the solution. These two effects by themselves are not large enough to account for the measured conductivity of the solutions. We speculate that there is high ionic mobility inside the polymer-surfactant complex that adds to the overall conductivity of the solution.