The chlorine peroxide molecule, ClOOCl is an important participant in the chlorine-catalyzed destruction of ozone in the stratosphere. Very few laboratory measurements have been made for the partitioning between monomer ClO and dimer ClOOCl at temperatures lower than 280 K. This paper reports absorption spectra for both ClO and ClOOCl when they are in equilibrium at 1 atm and temperatures down to 206 K. The very low ClO concentrations involved requires measuring and calibrating a differential cross section, Delta sigma(ClO), for the 10-0 band Of ClO. A third law fit of the new results gives K-eq = [(2.01 +/- 0.17) 10(-27) cm(3) molecule(-1)] e((8554 -/+ 21)K/T), where the error limits reflect the uncertainty in the entropy change. The resulting equilibrium constants. are slightly lower than currently recommended. The slope of the van't Hoff plot yields a value for the enthalpy of formation of ClOOCl at 298 K, Delta H-f(o) of 129.8 +/- 0.6 kJ mol(-1). Uncertainties in the absolute ultraviolet cross sections of ClOOCl and ClO appear to be the limiting factors in these measurements. The new K-eq parameters are consistent with the measurements of Santee et al.(42) in the stratosphere.