The interactions between oppositely charged polymer/surfactant mixtures at the air-water interface have been studied using neutron reflectivity techniques with supplementary surface tension measurements. The cationic polyelectrolyte poly(ethyleneimine)(PEI)/anionic surfactant sodium dodecyl sulfate(SDS) system is compared with a series of systems containing cationic ethoxylated poly(ethyleneimine)(PEI-EOn) with different degrees of ethoxylation of the PEI/anionic sodium dodecyl sulfate(SDS). The surface tension behavior and adsorption patterns for these systems show a strong dependence upon the solution pH. However, the SDS adsorption for PEI/SDS and PEI-EO1/SDS at the interface is unexpectedly most pronounced when pH is high (when polymer is essentially a neutral polymer). An obvious surfactant multilayer adsorption can be observed especially at intermediate concentrations at higher pHs 7 and 10 (area per SDS molecular ~20Ų). Based on fits to the neutron reflectivity data from different isotopic compositions the multilayer structure is consistent with a sandwich structure with an outer surfactant layer and a submerged micellar layer or defective bilayer. In contrast, for PEI-EO3/SDS and PEI-EO7/SDS, only monolayer adsorption is seen (area per SDS molecular ~50Ų). Furthermore, the pattern of SDS, PEI-EO3 and PEI-EO7 adsorption is indicative of a strong polymer/surfactant interaction at low pH, which decreases with increasing pH. Hence, the ethoxylation of the PEI indeed imposes a degree of surface activity on the polymer and modifies the nature of the surface adsorption.