The crystal structure of ClF was detd. at -188 Deg. ClF is monoclinic, space group P21/c, with a 6.378(1), b 4.247(1), c 6.425(1) .ANG., b 104.55(3) Deg; dc = 2.147,Z = 4, R = 0.0357, Rw = 0.0401 for 765 reflections. In the crystal individual ClF mols. form infinite planar ribbons along the screw axis, which are characterized by very short intermol. Cl...Cl contacts (3.070 .ANG.) between neighboring mols. An ab initio energy surface scan was performed for the ClF dimer by using the 6-311-G* basis set. Although the global energy min. of the ClF dimer was found for conformation A in the theor. study of the dimer, the T-shaped conformation (C,D) also has a min. which is energetically close to A, but is favored in the infinite ribbons of the crystal lattice. Some pos. charge is found in the prolongation of the F-Cl bond and some neg. charge perpendicular to the bond at the Cl atom, which holds the mols. together.