Molecular complexes between OCS and ClF, Cl(2), Br(2), or BrCl have for the first time been isolated in solid matrixes at low temperature, and characterized by their IR spectra. 1 : 1 adducts between OCS and ClF, Cl(2), or BrCl were formed by broad-band UV-visible photolysis of matrix-isolated FC(O)SCl, ClC(O)SCl or BrC(O)SCl, respectively. Co-deposition of gaseous mixtures of OCS and ClF or Br(2) diluted with Ar on a CsI window cooled to approximately 15 K led to the identification of 1 : 1 complexes, while similar mixtures of OCS and Cl(2) gave rise mainly to 1 : 2 species. The differences in the structures and stoichiometries of the molecular complexes may well be responsible for the different reaction channels observed for the photochemical matrix reactions between OCS and Cl(2) or Br(2). The structures, energy differences, vibrational spectra, and bonding properties of all the possible complexes formed between OCS and XY (XY = ClF, Cl(2), Br(2), or BrCl) have been studied with different theoretical approximations.