The crystal and molecular structure of 5-t-butyl-5-aza-2,8-dithia-1-stanna(II)bicyclo[3.3.01,5]octane [Sn(SC2H4)2NC4H9]2 have been determined from three-dimensional X-ray data. The crystals are monoclinic, space group P21/c, with cell dimensions: a 14.104(2), b 21.911(4), c 11.624(2) Å, β 98.73(1)°, Z = 6. The final R is 0.040 for the 4279 observed reflections. The unit-cell contains four asymmetric dimers (dimer a) and two centrosymmetric dimers (dimer b), each dimer forms a four-membered Sn2S2 ring in which the sulfur atoms are three-coordinated. The Sn---N distances in the two dimeric forms are significantly different (namely, 2.64 and 2.75 in dimer a, 2.97 Å in dimer b). The lengthening of the Sn---N distances involves the shortening of the bridging tin—sulfur bonds (2.99 and 2.79 in a, 2.71 Å in b). The coordination around the three different Sn atoms is hemispherical.