An experimental procedure has been developed for constructing pi–T diagrams (partial vapor pressure of the ligand-forming component–temperature) for luminescent metal complexes with symmetric organic ligands. The partial 8-hydroxyquinoline vapor pressure–temperature diagram has been constructed for tris(8-hydroxyquinoline)gallium electrophosphor (Gaq3) in the temperature range 300–617 K at 8-hydroxyquinoline vapor pressures 10–3–104 Pa, where the homogeneity ranges of different Gaq3 polymorphs have been determined. Structure-sensitive properties of crystalline materials can be tailored by changing synthesis conditions within the homogeneity range of a phase.