In order to elucidate the molecular structure of the intermediate compound obtained in the cyanide reaction of pyridoxal, an X-ray analysis was carried out of the crystal structure of its hydrobromide (CgHloO3Nz.HBr.H20). Crystals of the hydrobromide are monoclinic, space group P21/a and the lattice parameters are a= 10.15, b= 17.70, c=6.83 A, t = 110-8 °. There are four molecules in the unit cell. The structure was solved by the heavy atom method using three-dimensional Fourier and difference Fourier syntheses. The refinement was carried out by the block-matrix least-squares calculations including anisotropic temperature factors. The final R value for 1306 non-zero reflexions was 0.095. The molecule is almost planar, only the oxygen atom of the hydroxymethyl group being out of the bicyclic plane. Molecules are bound together through four kinds of hydrogen bonds. The molecular structure was found to be of the aminofuran type, the exact name of the compound being 2-amino-3hydroxy-4-hydroxymethyl-7-methyl-d2-furo[2,3-c]pyridine hydrobromide.