The phase behavior and microstructure of aqueous mixtures of n-octyl-β-D-glucoside (C 8βG 1) and triethylene glycol mono-ra-octyl ether (C 8E 3) is presented. C 8βG 3 forms a one-phase micellar solution in water at surfactant concentrations up to 60 wt %, wheres mixtures with C 8E 3 show a liquid-liquid phase transition at low surfactant concentration. The position of this phase boundary for mixtures can be rationally shifted in the temperature-composition window by altering the ratio of the two surfactants. Small-angle neutron scattering is used to determine the size and shape of the mixed micelles and to characterize the nature of the fluctuations near the cloud point of the micellar solutions. The C 8βG 3/C 8E 3 solutions are characterized by concentration fluctuations that become progressively stronger upon approach to the liquid-liquid phase boundary, whereas micellar growth is negligible. Such observations confirm previous views of the role of the surfactant phase boundary in tuning attractive micellar interactions, which can be used effectively to change the nature and strength of interparticle interactions in colloidal dispersions. Colloidal silica particles were then added to these surfactant mixtures and were found to aggregate at conditions near the cloud point. This finding is relevant to current strategies for protein crystallization.