Specular neutron reflection and surface tension have been used to investigate the composition and structure of the surfactant-polymer mixture of sodium dodecyl sulfate, SDS, and the cationic polymer poly(dimethyldiallylammonium chloride) at the air-water interface. The variation of surface tension with SDS concentration shows a complex behavior, with a marked increase between the concentrations normally associated with the critical aggregation concentration and the critical micellar concentration. The neutron reflectivity measurements show that this change in surface tension is associated with changes in the amount of SDS and polymer at the interface. The changes are attributed to the competition between the formation of surface and solution surfactant-polymer complexes.