The solution phase behaviour of poly(2, 6-diphenyl-p-phenylene oxide) (PPPO) is investigated by a combination of turbidimetry, infrared spectroscopy, dynamic light scattering and densitometry, combined with calorimetry and X-ray scattering. We select dichloromethane (DCM) and heptane as, respectively, representative good and poor solvents for the polymer. This ternary system results in a miscibility gap which can be utilised for the design and fabrication of PPPO porous materials, membranes and scaffolds via phase inversion. We establish the phase diagram and resolve the kinetic solidification condition arising from the intersection between the coexistence and glass transition curves. PPPO exhibits a high 230 ∘C and is found to crystallise at 336 ∘C, and melt at 423, 445 ∘C with a double endotherm. The kinetics of demixing and (buoyancy-driven) stratification are quantified by optical imaging and the PPPO-rich phase analysed by SAXS/WAXS to resolve both amorphous and crystalline phases. Equipped with this knowledge, we demonstrate the controlled formation of nodular, bicontinuous and cellular morphologies by non-solvent induced demixing.