Self-assembling polymer-grafted nanoparticles (PGNs) are advancing roll-to-roll manufacturing of photovoltaics, microsensors, opto-electronics, memory, and multifunctional coatings. Architectural features of the polymeric canopy (e.g. grafting density, chain length) not only determine processibility and assembly, but also robustness, such as mechanical toughness and dielectric breakdown strength. To control PGN assembly, and therefore tailor properties, a detailed understanding of solution phase behavior is imperative. However, while phase properties of macromolecules have been extensively explored, comparable knowledge of PGN solution phase behavior is lacking and fails to elucidate the role of the PGN constituents. Here we map the phase space of polystyrene-grafted gold-nanoparticles in cyclohexane by establishing the upper critical solution coexistence curve via UV-vis spectroscopy, dynamic light scattering, and small angle X-ray scattering. Increasing the molecular weight of the polystyrene graft (20 → 50 kDa) advances the coexistence curve to higher temperatures (Tc ~ 12 → 20 oC @ 10 nM (φ ~3e-5)) and increases the estimated theta temperature (θ ~ 25 → 27 oC). Considering the well-established phase behaviors of structured macromolecules, the coexistence curves and theta temperatures of PGNs are less than those of polystyrene stars (θ ~ 30 oC) and linear chains (θ ~ 33 oC). Using a TEM solution cell the impact of the proximity to the coexistence curve (e.g. spinodal decomposition v. nucleation and growth) on the assembly process is demonstrated. Such insight refines processing protocols for slot-die coating techniques, allowing for rapid self-assembly of large-area ordered PGN monolayers directly on solid substrates.