The interactions, in aqueous medium, between hydrophobically end-capped poly(ethylene oxide) (PEO) and sodium dodecyl sulfate (SDS) have been investigated by means of tensiometry, 13C NMR spectroscopy and small-angle neutron scattering (SANS). Besides the association of SDS with the ethylene oxide backbone of associative polymer (AP), revealed by 13C NMR, SDS interacts with hydrophobic endcaps leading to the formation of mixed aggregates. At low SDS concentration, the formation of flowerlike mixed aggregates is governed by AP. Despite the poor affinity between hydrocarbon and fluorocarbon chains, uncooperative binding was detected between SDS and fluorinated AP end groups. With a further increase of SDS concentration, the critical aggregation concentration (cac) marks the onset of SDS cooperative binding with AP end-caps. This process driven by SDS leads to the formation of micellar aggregates with a necklacelike structure. Due to the SDS/AP complex formation, the surfactant aggregation number determined by SANS is lower than that observed in pure water.