The tetravalent SeCl₄ and SeBr₄ are reduced in the presence of thioether ligands L (SMe₂, tht) or L–L (MeS(CH₂)_nSMe (n = 2 or 3), o-C₆H₄(SMe)₂) in MeCN solution at 0 °C, forming Se(II) thioether complexes, including the crystallographically characterised halo-bridged chain polymers [SeX₂(SMe₂)] (X = Cl or Br), molecular trans-[SeX₂(tht)₂], cis-[SeBr₂{MeS(CH₂)₂SMe}] and the thioether-bridged polymer [SeBr₂{MeS(CH₂)₃SMe}], as the main products, together with halogenated ligand. The [SeX₂(L)₂] and [SeX₂(L–L)] complexes are all based upon distorted square planar coordination, with two Se-based lone pairs assumed to occupy the (vacant) axial sites, and Se–S bond distances of ca. 2.4–2.6 Å. The 1:1 species [SeX₂(SMe₂)] are T-shaped with trans X groups and weak intermolecular SeX contacts. The SeCl₂-thioether complexes are less stable than the bromides, both in solution in CH₂Cl₂ and as solids at ambient temperature. Reaction of SeBr₄ with o-C₆H₄(SMe₂)₂ leads to the red complex cis-[SeBr₂{κ¹-o-C₆H₄(SMe)₂}₂] as the major product; together with a minor (yellow) product formed via bromination of the aromatic ring, [SeBr₂{4-Br-1,2-(SMe)₂-C₆H₃}₂]. The crystal structure confirms a V-shaped SeBr₂ unit with long (weak) κ¹-interactions to one S donor (meta to the Br) from two brominated ligands – an extremely rare coordination mode for an o-phenylene dithioether. Similar reaction of o-C₆H₄(SMe₂)₂ with SeCl₄ leads to several species, including monosulfonium cation, [1]⁺ formed by coupling of one thioether group to the C4-position of the phenylene backbone in an adjacent molecule, confirmed crystallographically. Carbon-sulfur coupling is also evident in the reaction of SeX₄ with o-C₆H₄(CH₂SMe)₂, leading to two related cyclic sulfonium species, [2]⁺ and [3]⁺, which were structurally characterised as [SeBr₄]²⁻ and [Se₂Cl₆]²⁻ salts respectively. Reaction of SeX₄ with SeMe₂ leads to halogenation of the ligand to form Me₂SeX₂ and reduction of the SeX₄ to elemental selenium.