Interaction between the thermoresponsive polymer poly(N-isopropylacrylamide) (P-NIP) and sodium dodecyl sulfate (SDS) both above and below its phase transition temperature was examined under dilute conditions. Above the lower critical solution temperature (LCST) of P-NIP (32 degrees C), 0.01 wt % P-NIP specifically interacted with 1.0 x 10(-5) mol/L SDS to form a precipitate. However, when SDS was added at concentrations above or below 1.0 X 10(-5) mol/L, the P-NIP solution remained clear above the LCST. A fluorometric probe, N-phenyl-naphthalene, indicated that the hydrophobicity of the aggregates composed of P-NIP and SDS changed at an SDS concentration of 1.0 x 10(-5) mol/L. Although the hydrophobicity of the precipitate was similar to that of P-NIP alone at less than 1.0 x 10(-5) mol/L, it approached that of SDS homomicelles as the SDS concentration increased above 1.0 X 10(-5) mol/L. Dynamic light scattering and turbidimetry studies showed no P-NIP phase transition above an SDS concentration of 1.0 x 10(-5) mol/L, which is much lower than the reported critical association concentration (CAC) of SDS with P-NIP. This indicates that P-NIP interacted with SDS above the LSCT at much lower SDS concentration than the reported CAC.