The interactions between poly(ethylene oxide) (PEO) and dodecyl sulfate with different monovalent metal cations (MDS, M = Li+, Na+, Cs+) and dodecyltrimethylammonium bromide (C(12)TAB) have been studied by flow microcalorimetry. The enthalpy change on dilution of a solution of surfactant and polymer with a solution of just polymer as a function of final concentration of surfactant has been measured directly for each of the surfactants. From the variation of enthalpy change with final concentration the thermodynamic parameters of micellization and aggregation have been determined. The enthalpies of binding of MDS micelles to PEO were found to be -3.7, -2.2, and -3.4 +/-0.5 kJ mol(-1) for LiDS, NaDS, and CsDS, respectively, at 0.1 wt % PEO. When combined with the free energies of binding, determined from the critical micelle and critical aggregation concentrations, which show a much greater variation with counterion, with corresponding values of -3.9, -3.1 and -0.7 kJ mol(-1), significant differences in the entropy of binding were found between the different counterions. Larger values of the entropy for LiDS and NaDS may result from the displacement of a larger number of water molecules when polymer binds to the micellar surface than in the case of CsDS.