Two new phases of zirconium tetrakis(8-hydroxyquinolinolate) (Zrq₄) have been synthesised and characterised by single crystal X-ray diffraction. Their electrical, electronic, optical and thermal properties have been studied. Their electron transporting characteristics have been investigated in organic light emitting devices where the two phases show remarkable differences in performance. One of the forms (designated α-Zrq₄) gives significantly lower operating voltage, higher efficiencies and longer lifetime than the other (designated β-Zrq₄) in organic light emitting devices.