Highly negatively charged polystyrene particles suspended in aqueous nonionic surfactant solutions undergo a phase transition when the solution is in close proximity to the cloud curve of the binary nonionic surfactant-water mixture. This transition is present only on the surfactant-rich side of the critical point of the binary mixture and is characterized by the presence of brilliant colors in the solution. These colors indicate the formation of polycrystalline aggregates of the polystyrene particles. This particle phase behavior is similar to the aggregation behavior observed for particles dispersed in aqueous solutions of 2,6-lutidine and its analogs. The observed behavior is best interpreted in terms of the appropriate ternary phase behavior and results from immiscibility between the particles and one of the liquid components. Although the details and origins of this unfavorable interaction vary from system to system, the same general pattern of phase behavior is observed in many mixed colloidal systems.