Hexamethylenediamine dihydrobromide, C6H18N2Br2, is monoclinic, space group P21/c, with four molecules per unit cell. Only minute needle crystals were available, but the structure factors of the principal zone, (0kl), have been measured. A Patterson analysis determines the positions of the eight bromine atoms in this projection without ambiguity. This is followed by a two-dimensional Fourier synthesis from which the approximate positions of all the atoms can be estimated. The results are consistent with an extended chain structure of normal bond distances (1.4–1.6 A.), and with Br—N distances lying between 3.2 and 3.5 A. These results provide a basis for a more accurate determination of the structure of the isomorphous dihydrochloride.