The crystal and molecular structures of the diethyltin dihalides, R2SnX2[(1) X = Cl, (2) X = Br, (3) X = I], have been determined from diffractometer data. Crystal oarameters are: (1), monoclinic, space group P21/c, a= 9.677(3), b= 9.835(2), c= 9.243(3)Å, β= 102.73(2)°, Z= 4. 811 observed reflections. R 0.047; (2), orthorhombic, space group C2221, a= 9.786(2), b= 10.006(3), c= 9.494(2)Å, Z= 4. 289 observed reflections, R 0.058; (3), orthorhombic, space group Pbcn, a= 13.469(4), b= 6.385(2), c= 13.733(4)Å, Z= 4, 342 observed reflections, R 0.049. In (2) and (3), the individual molecules have crystallographic symmetry 2. All three compounds form chains of interacting molecules with each tin atom forming four primary and two secondary bonds such that the geometry may be described as intermediate between tetrahedral and octahedral. The lengths of the secondary bonds in (1) and (2) are 3.461(4) and 3.777(4)Å, 0.39 and 0.17 Å less than the sum of the respective van der Waals radii, with C–Sn–C angles opened to 134.0(6) and 135.9(10)°. For (3) the secondary bond distance is 4.284(5)Å, longer than the sum of the van der Waals radii, although the C–Sn–C angle [130.2(11)°] shows that the interaction is stall stereochemically important. The packing in (3) also differs from that in (1) and (2). The lengths of these secondary bonds are compared with other known examples in organotin(IV) chemistry, and factors influencing the strength of the Interactions are discussed.