Lattice Monte Carlo simulations have been performed to study the equilibrium phase behavior of model amphiphilic solutions in the presence of hybrid organic-inorganic particles. According to the nature of the particles and the surfactant concentration, these systems are able to phase separate into a dilute phase containing a high solvent content and a concentrated phase containing mainly the surfactant and hybrid particles. In some cases, the concentrated phase shows the presence of structured liquid crystal phases, such as hexagonally ordered cylinders or lamellae. The solubility in the solvent and the nature of the organic group belonging to the hybrid particles are key aspects affecting the driving force for the phase separation, hence the concentration of surfactant in the phases at equilibrium. Biphasic regions have also generally been detected by applying the quasichemical theory, which employs a lattice-based mean field approximation. Good quantitative agreement with the results obtained from the simulations was typically found, especially when no self-assembly into ordered aggregates was observed. © 2007 American Chemical Society.