Self-diffusion constants of amphiphilic molecules in D2O solutions of mixed poly(oxyethylene)-polydimethylsiloxane diblock copolymer (POE-PDMS, SimC3EOn) and poly(oxyethylene) dodecyl ether (C12EOn) were measured by pulsed-field-gradient NMR method. In the D2O/ Si25C3EO51.6/C12EO8 or D2O/Si52C3EO51.6/C12EO8 systems, small andlarge micelles coexist in a wide range of SimC3EO51.6 fractionintotal amphiphiles, whereas such a coexisting phenomenon does nottakeplace in the D2O/Si5.8C3EO51.6/C12EO8 system. The coexisting phenomenon also takes place in the D2O/Si25C3EO51.6/Cl2EO5 system although the range of mixing fraction is limited. By obtaining each contribution of surfactant and copolymer molecules to the attenuation decay of the echo signal from the proton of the poly(oxyethylene) chain, we could evaluate thecomposition of the mixed micelles in the D2O/Si25C3EO51.6/Cl2EO8 system. The copolymer content in the mixedmicelle increases proportionally to the copolymer mole fraction in the aqueous solution. From the series of self-diffusion measurements, we can conclude that the miscibility Of SimC3EOn and C12EOn in aqueous micelles becomes poor and the coexisting phenomenon takes place when the PDMS chain becomes much longer than the dodecyl chain of C12EOn or the POE chain Of C12EOn becomes long. Furthermore it is also revealed that very few silicone copolymer molecules can be incorporated in small surfactant micelles. (c) 2006 Elsevier Inc. All rights reserved.