The binding of SDS to pluronic F127 was studied using a SDS surfactant; selective electrode via electromotive force, isothermal titration calorimetry, and light scattering. Ata concentration of 0.5% w/v the block copolymer F127 exists as an equilibrium mixture of micelles and monomers at 35 degreesC. When SDS is gradually added to this solution, binding of SDS to the F127 micelles takes place even at the lowest; measured SDS concentration (1 x 10(-5) mol dm(-3)). Initially F127/SDS mixed micelles are formed and the size of these micelles remains constant until similar to5 x 10(-5) mol dm(-3) of SDS has been reached. At a total SDS concentration of 5 x 10(-5) mol dm(-3) there are similar to6 SDS monomers in an aggregate containing 69 F127 monomers. Further addition of SDS results in a dramatic breakdown of the F127 rich mixed micelles into smaller aggregates. During this process SDS continually binds to the F127 micelles forming mixed micelles which simultaneously break down to smaller aggregates and also become richer in SDS. This process continues until the monomer SDS concentration reaches a value of 3.5 x 10(-4) mol dm(-3) which is the thermodynamic condition for the onset of the binding of SDS micelles on unassociated F127 oligomers. This occurs until the total SDS concentration reaches 1 x 10(-3) mol dm(-3) which precedes the point where all the F127/SDS mixed aggregates are broken down (3 x 10(-3) mol dm(-3)). Therefore in the SDS concentration region I x 10-3 to 3 x 10-3 mol dm-3 two simultaneous binding processes take place (1) SDS forms mixed micelles with F127 and these micelles break down to smaller aggregates, and (2) SDS micelles bind to unassociated F127 oligomers. Further addition of SDS in excess of 3 x 10-3 mol dm(-3) results in the formation of more and larger SDS micelles bound to individual F127 monomers until all the unassociated F127 oligomers available for binding become fully saturated with bound SDS micelles at similar to0.1 mol dm(-3) added SDS. At this limiting point the complexes contain, on average, one SDS micelle per F127 monomer. No F127 aggregates appear to exist when the SDS concentration reaches or exceeds 3 x 10(-3) mol dm(-3).