The dimeric ClO has been generated in a gaseous discharge-flow system at 220 K and 10-30 Torr by reacting atomic Cl with O3, ClOCl or OClO, and reacting atomic O with ClOCl or OClO, in a stream of Ar. The gas mixture in the flow tube was sampled through a pinhole and condensed on a CsI window maintained at 12 K. The infrared absorption spectrum of the matrix formed after 30-120 min deposition was then recorded. Absorption lines at 752.6, 649.8, and 647.6 cm-1 have been assigned to ClOOCl, and previous assignments for dimeric ClO seem to be in error. The results are in excellent agreement with recent ab initio calculations. Under our experimental conditions, no other isomer of Cl2O2 was observed.