Samples formed by codepositing S 2 from a superheater source with Ar/O 2 on a 10 K substrate exhibit very strong 1403.0-cm -1 and weaker 725.5-cm -1 infrared absorptions. Photolysis decreases these bands slightly and produces SO 2 and S 2 O. The infrared bands show 18 O 2 and 34 S 2 shifts appropriate for O-O and S-S fundamental vibrations, and triplet absorptions in mixed isotopic experiments suggesting two equivalent O and two equivalent S atoms in the product complex. Similar results were obtained for Se 2 and O 2 ; new absorptions appeared at 1404.5 and 391 cm -1 . Ab initio calculations at the SCF, CISD, and CCSD levels of theory failed to find a complex with the observed spectroscopic properties. However, calculations with the BP density functional characterized a singlet (S 2 )(O 2 ) parallelogram structure bound by 15.6 kJ/mol relative to triplet S 2 and O 2 with the O-O stretching frequency red-shifted 187 cm -1 and the S-S fundamental blue-shifted 20 cm -1 . This weakly bound (S 2 )(O 2 ) complex is chemically intermediate between the unstable O 4 and stable S 4 molecules. The argon matrix has made possible the formation of the weak (S 2 )(O 2 ) complex and DFT with the BP functional has characterized this weak charge-transfer interaction.