The self-association of copolymers of N-isopropylacrylamide and N-n-octadecylacrylamide (Pnipam-C18) in aqueous solutions was studied by means of time-resolved fluorescence quenching. The discrete domains consist of several polymer chains interacting through their hydrophobic side chains, since the number of aliphatic side chains involved in the microdomain formation (aggregation number) is larger than the number of aliphatic side chains per polymer. By means of titration microcalorimetry, the interaction of the copolymer with surfactants was studied. Strong association between the copolymer and the cationic surfactants N-cetylpyridinium chloride (C1GPyCl) and cetyltrimethylammonium bromide (CTAB) occurs by partitioning of the surfactants in a noncooperative mechanism. Prior to mixed-micelle formation, individual surfactant molecules adsorb to collapsed polymer coils as can be seen from the large exothermal contribution in the enthalpy curves which result from microcalorimetric titration of surfactant into aqueous Pnipam-C18 solutions.