Abstract: There is growing interest in the formulation and application of nano colloids + non-adsorbing polymer in fields of nanomedicine, hydrocarbon production, and environmental science. Successful applications rely on a detailed understanding of the related fluid thermophysical and thermochemical properties. Polymer + solvent binary mixtures exhibit liquid-liquid phase behavior at low temperature. This behavior, driven by repulsive interactions between the globular conformation of the polymer, possess an UCEP temperature because the polymer undergoes a conformational change. From a Gibbs free energy perspective, at temperatures slightly above the UCEP temperature, a single-phase liquid is marginally stable. This phase behavior is driven by entropically favourable molecular interactions between polymer in the coil conformation and solvent molecules. Phase diagrams for nanoparticle + non-adsorbing polymer + solvent ternary mixtures typically exhibit colloid gas, G (a liquid comprising largely polymer and solvent), a colloid liquid, L (a liquid comprising largely solvent and colloid) and a colloid phase, C (a largely nanoparticle rich phase) phases. The phase behaviors GC, GL, LC and GLC at equilibrium are driven by depletion flocculation (a surface phenomenon). This work focuses on the interaction between these two phenomena – depletion flocculation and molecular liquid-liquid phase behavior based on the behavior of polystyrene + cyclohexane + nano silica particles. For example, above the UCEP temperature, even a trace mass fraction of silica nanoparticle (