Analysis by gas chromatography coupled with mass spectrometry (GC-MS) confirmed the presence of ADE, leading us to conclude that we indeed obtained a new polymorph of ADE. Furthermore, 1 H- C cross-polarization magic angle spinning nuclear magnetic resonance spectra (CPMAS) of two phases were similar and consistent with that conclusion (ESI Fig. ). To obtain single crystals for the purpose of structural studies via X-ray diffraction, we recrystallised the sample that was heated to 115 °C in several solvents, including diethyl ether, n-pentane, and THF (ESI Fig. ). Pleasingly, crystals grown in THF corresponded to the newly discovered form. We solved the crystal structure of the new phase from singlecrystal X-ray diffraction data (ESI Table ) and confirmed that ADE has at least two polymorphic forms, a room-temperature form named polymorph α, and a high-temperature form named polymorph β (Fig. ). Polymorph α crystallises in a monoclinic P2/n space group with four molecules of ADE in the unit cell, while polymorph β crystallises in a triclinic 𝑃1 ̅ space group with two molecules of ADE in the unit cell. Both polymorphs exhibit comparable C-O bond distances but differ slightly in C-O-C angles, which correspond to 128.109(2)° in polymorph α and 127.821(2)° in polymorph β. Crystal packing in α and β is similar and dominated by close C-H⋯H-C contacts.