Diphenyl-1,3,4-oxadiazole (DPO) crystallization experiments from solutions clearly reveal the polymorphism of the substance. Besides the formerly known centrosymmetric monoclinic structure with space group P2 1 /c (DPO I) a new monoclinic structure with the non-centrosymmetric space group Cc is found (DPO II): a=2.4134(4)nm, b=2.4099(3)nm, c=1.2879(2)nm, β=110.048(3) o , and V=7.0363(17)nm 3 . The asymmetric unit contains six independent molecules in a complex packing motif. A re-determination of the crystal structure of DPO I at room temperature gives lattice parameters a=0.51885(6)nm, b=1.8078(2)nm, c=1.21435(14)nm, β=93.193(3) o , and V=1.1373(2)nm 3 . X-ray measurements at 363K show a significant increase of the unit cell volume by 1.6%. Differences between both structures concerning morphology and characteristic Raman bands are outlined in detail. DSC investigations show an irreversible transition from DPO I to DPO II at 97 o C. DPO II does not show any transition in the temperature range up to the melting point at 141 o C. The non-centrosymmetric DPO II structure shows triboluminescence.