The adsorption of the polymer/surfactant mixture of ethoxylated poly(ethyleneimine) (PEI) and sodium dodecyl sulfate (SDS) at the air-water interface has been studied by specular neutron reflectivity and surface tension. PEI is a strong polyelectrolyte at low pH and behaves like a neutral polymer at high pH. The variation in the concentration dependence of the SDS adsorption at the interface and the corresponding surface tension behavior, in the presence of 20 ppm PEI and at different pHs, reflect the changing nature of the polymer with pH. At SDS concentrations of < 0.5 mM, the SDS adsorption increases with decreasing pH, whereas for SDS concentrations of >0.5 mM it is essentially independent of pH. This change in adsorption behavior with concentration corresponds roughly to the concentration at which the PEI no longer adsorbs at the interface. Furthermore, at the lower SDS concentrations PEI adsorbs at the interface, and the PEI adsorption increases with decreasing pH. The pattern of SDS and PEI adsorption is indicative of a strong polymer/surfactant interaction at low pH, which decreases with increasing pH. The surface tension behavior at different pHs shows neither the classical dependence on surfactant concentration of weakly interacting polymer/surfactant mixtures, such as poly(vinylpyrrolidone)/SDS, nor the more complex behavior of charged polymer/oppositely charged surfactant mixtures. Although the ethoxylation of the PEI avoids phase separation in concentrated PEI/surfactant solutions, the adsorption data and surface tension demonstrate that it provides, in addition to pH, a modification to the effect of the strength of the interaction between the PEI and the SDS. In addition to the increasingly favorable interaction between PEI and SDS with decreasing pH, there is also an element of competitive adsorption arising from the inherent surface activity of the PEI.