Block copolymers of the type poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) [EO(x)PO(y)E0(x)] form micelles with a hydrophobic core of propylene oxide and a palisade layer of the ethylene oxide. We studied the influence of sodium dodecyl sulfate (SDS) on the aggregation behavior of F127 (EO(97)PO(69)OE(97)) by static and dynamic light scattering, electric birefringence, and calorimetric methods. We observed that for block copolymer solutions in the range of 1-5 % and increasing concentration of SDS the light scattering passed through a deep minimum, the electric birefringence passed over a maximum, and the endothermic peak of the DSC signals which is due to the micellization of the block copolymer disappeared. The results show that SDS binds to monomers of F127 and thereby suppresses completely the formation of F127 micelles. At saturation about six molecules of SDS bind to one F127 molecule. The birefringence data indicate that the F127/SDS complex exists in a more or less extended conformation and not as a coil. The binding of SDS on F127 was also confirmed by surface tension measurements. These measurements show that the surface active block copolymers are replaced from the interfaces by SDS molecules. The binding of SDS on F127 begins at concentration far below the critical micelle concentration of SDS.