The fluorescence anisotropy of perylene and the native fluorescence of Igepal CO-630 and Triton X-114 were used to study the intramicellar fluidity of mixed micellar solutions containing these nonionic surfactants and added sodium dodecyl sulfate. While the addition of small amounts of SDS significantly increased cloud points, the internal structure of the nonionic micelles remained essentially intact. At higher SDS concentrations, the perylene anisotropy decreased smoothly, indicating the movement of the probe away from the palisade layer of the micelle and/or an increase in the core fluidity. Mixed micelles containing only nonionic surfactants showed that the core fluidity was higher in the mixture than would be expected from a simple additive model of two probe environments. Mixed micelles with intermediate SDS concentrations showed interesting phase behavior, in which a "preclouded" colloidal phase preceded conventional clouding. Using solution turbidity as a delineator of phase behavior, preclouding was found to be sharply dependent on the proportion of added SDS.