FROM a purified specimen of ‘hexuronic acid’ (identified by Szent-Gyorgyi with vitamin C) available in this laboratory I have been able to obtain sufficiently good crystals to carry out an X-ray examination by the single-crystal rotation method. The substance is monoclinic sphenoidal, with a = 17.71, b = 6.32, c = 6.38 A., and I² = 102½°, while the space-group is , since the only true halving is (0k0) absent when k is odd. The density of the crystals was determined by the flotation method to be 1.65 gm./c.c., so that there are four molecules of C6H8O6 in the unit cell. Since has only twofold symmetry, a pair of molecules must be associated to form the asymmetric crystal-unit. (This does not necessarily imply polymerisation.)