The crystal structures of a-F 2 and b-F 2 have been reinvestigatedu sing neutron powder diffraction. For the low-temperature phase a-F 2 ,w hich is stable below circa 45.6 K, the monoclinic space group C2/c with lattice parameters a = 5.4780(12), b = 3.2701 (7), c = 7.2651(17) , b = 102.088(18)8, V = 127.26(5) 3 , mS8, Z = 4a t1 0Kcan now be confirmed.T he structure model was significantly improved, allowed forthe anisotropic refinement of the Fatom, anda n FÀFbond length of 1.404(12) was obtained, which is in ex-cellent agreement with spectroscopic data and high-level quantum chemical predictions.T he high-temperature phase b-F 2 ,s table between circa 45.6 Ka nd the melting point of 53.53 K, crystallizes in the cubic primitive space group Pm3 n with the lattice parameter a = 6.5314(15) , V = 278.62(11) 3 , cP16, Z = 8, at 48 K. b-F 2 is isotypic to g-O 2 and d-N 2 .T he centres of gravity of the F 2 molecules are arranged like the atoms in the Cr 3 Si structure type.[a] Dr.S.I.I vlev,Dr. M. Conrad, Prof.