In our study we determined the diffusion coefficient of Tween 20 micelles at different surfactant concentrations (10(-4)-10(-2) M) via fluorescence correlation spectroscopy. The hydrophobic fluorescent dye 9,10-bis(phenylethynyl)anthracene, located in the micellar phase, was used as marker for the diffusion of Tween 20 micelles. The hydrodynamic radius of the micelles, Rh, was estimated from the diffusion coefficient and compared with literature values from previous studies. (C) 2009 Elsevier Inc. All rights reserved.