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The crystal structure of 5-amino-2-thiol-l,3,4-thiadiazole (C2H3NaS2) has been determined by threedimensional Patterson and Fourier methods and refined by block-matrix least-squares calculations.The crystals are monoclinic, space group P21/c with four molecules in a unit cell of dimensions a = 6.167, b= 10.048, c=9-007,~, fl=116.8 °. The final R index is 0.106 for 796 observed reflexions, collected with Cu K0c radiation. The molecule may be regarded as being planar since the deviations of the atoms from the mean plane of the molecule are all less than 0.02/~. Bond lengths indicate that the compound exists in the thione rather than the thiol form. The 1,3,4-thiadiazole ring possesses a fair degree of aromaticity with a localized C(1)=N(1)double bond of length 1.305 (6) ~. Other C-N bond lengths are 1.334 and 1.339 ~ with a mean estimated standard deviation of 0.009 A. The N-N bond length is 1.382 (8) A,. The cyclic C-S distances are 1.746 and 1.748 A and the exocyclic C-S bond length is 1"678 A, the mean e.s.d, of these three bonds is 0.006 ~. Of particular interest is the short S-S intramolecular contact of 3.008 A which is approximately 0.5 A shorter than the sum of the Pauling van der Waals radii. There is a reasonable probability that hydrogen bonds of type N-H. • • S and length 3"32 A exist, joining pairs of molecules across centres of symmetry.