A series of di(para-halophenyl)diselenides, (p-XC₆H₄)₂Se₂ (X = F, Cl) have been reacted with three equivalents of SO₂Cl₂ or Br₂, leading to the formation of selenium(IV) RSeX₃ compounds. The structures of (p-FC₆H₄)SeX₃ (X = Cl, Br) have been determined, and both exhibit a dimeric RSeX₂(μ-X)₂RSeX₂ structure consisting of two “saw-horse” (p-FC₆H₄)SeX₃ units linked by two halide bridges, with an overall square pyramidal geometry at selenium. In both structures all the selenium and halogen atoms are planar, with both aryl rings located on the same side of the Se₂X₆ plane (cis-isomer). The structure of (p-ClC₆H₄)SeCl₃ also adopts a planar dimeric structure, however both cis- and trans-dimeric molecules are co-crystallised in the unit cell. In contrast, the structure of (p-ClC₆H₄)SeBr₃ adopts a folded cis-dimeric structure due to steric constraints. Secondary SeX interactions to the “vacant” sixth coordination site at selenium are a feature of most of these structures, but are most prominent in the folded structure of (p-ClC₆H₄)SeBr₃. A re-examination of the PhSeBr/PhSeBr₃ system resulted in the isolation of crystals of a second structural form of “PhSeBr”. The structure of Ph₂Se₂Br₂ consists of two PhSeBr units linked by an elongated Se–Se bond of 2.832(4) Å, and longer secondary SeBr interactions of 3.333(4) Å to form a chain structure. Further weak SeBr and BrBr interactions are present, which form loosely linked rippled sheets of selenium and bromine atoms, similar to the sheets observed for the tetrameric form, Ph₄Se₄Br₄.