Nitroxides radicals are attractive tools for study in biochemistry and materials science. A polymorphic crystal of a nitroxide radical, 4,4,5,5-tetramethyl-2-phenylimidazoline-1-oxyl 1-oxide was prepared and structurally characterised. The compound crystallised in the monoclinic, space group C2/c with a = 11.170(6) Å, b = 10.417(5) Å, c = 10.995(5) Å, β = 108.059(9)°, V = 1216.3(10) Å, C13H 17N2O2, Mr = 233.29, Dc= 1.274 g cm-3, μ (MoKα) = 0.087 mm-1, Z = 4 and F(000) = 500. The structure was solved by direct methods and refined by leas...