The ligand 2-(3,3-dimethyl-2-thiabutyl)pyridine (L) and its protonated cation, 2-(3,3-dimethyl-2-thiabutyl)-pyridinium ion (LH+), have been used to prepare the copper(I) complexes [CuLnBr](n= 1 or 2) and [Cu(LH)X2](X = Cl or Br). Electronic and i.r. spectroscopic data for these complexes are presented, and the chemical relationships between them discussed. The crystal and molecular structures of [CuL2Br] and [{Cu(LH)Br2}2] have been determined by single-crystal X-ray diffraction techniques, from diffractometer data. Crystals of [CuL2Br] are monoclinic, space group P21/n, with a= 16.927(3), b= 8.867(1), c= 15.371(2)A, β= 100.61(1)°, and Z= 4. Crystals of [{Cu(LH)Br2}2] are also monoclinic, space group P21/n, with a= 15.700(3), b= 9.038(3), c= 9.926(2)A, β= 98.66(1)°, and Z= 2. After full-matrix least-squares refinement final R values are 0.066 (for 2 032 observed reflections) and 0.047 (for 1 538 observed reflections) respectively. The complex [CuL2Br] is monomeric, the distorted tetrahedral geometry round the copper atom being completed by a bromide ion [Cu–Br 2.424(2)A], the pyridyl nitrogen and thioether sulphur of a bidentate ligand L [Cu–N 2.109(9), Cu–S 2.357(3)A], and the thioether sulphur of a unidentate ligand L [Cu–S 2.310(3)A]. The complex [{Cu(LH)Br2}2] exists as a centrosymmetric dimer with the distorted tetrahedral copper(I) centres dibromo-bridged [Cu–Br 2.597(1)A]. The two remaining co-ordination positions round each copper are occupied by a terminal bromide ion [Cu–Br 2.363(1)A] and the thioether sulphur [Cu–S 2.276(2)A] of an LH+ cation.