Diffusion of organic dyes in neat room temperature ionic liquid (RTIL) and RTIL-mixed micelle has been studied by fluorescence correlation spectroscopy (FCS). We have selected two RTILs, 3-pentyl-1-methyl imidazolium bromide ([C5C1Im][Br]) and the corresponding tetra-fluoroborate ([C5C1Im][BF 4]). Diffusion coefficients (Dt) of three organic dyes-DCM (neutral), C480 (neutral), and C343 (anionic)-in these RTILs are ∼100 times slower compared to water. This indicates very high viscosity of the RTILs. In contrast to water, the Dt in RTIL exhibits a wide distribution which suggests the presence of heterogeneity (nanoscale organization). The presence of ions in the RTILs markedly affects diffusion in the RTILs. Dt's of C480 (neutral) and C343 (anionic) are very similar in water but in RTILs the ionic dye C343 diffuses 1.7 times slower than neutral C480. This is attributed to the electrostatic force exerted by the ions in the RTILs. In the giant (∼2-4 μm) [C5C1Im][Br]-triblock copolymer (P123) mixed micelle D t of DCM, C480, and C343 are found to be 7, 15, and 7 μm 2s-1, respectively. The results are compared with those in P123 micelle and gel. © 2011 American Chemical Society.