Abstract. The thermal equilibrium constant between the chlorine monoxide radical (ClO) and its dimer, chlorine peroxide (ClOOCl), was determined as a function of temperature between 228–301K in a discharge flow apparatus using broadband UV absorption spectroscopy. A third law fit of the equilibrium values determined from the experimental data provides the expression: K_eq = 2.16×10⁻²⁷ e^{(8533±25K/T)}cm³molecule⁻¹. A second law analysis of the data deviates minimally: K_eq = (2.06±1.27)×10⁻²⁷ e^{(8546±123K/T)}cm³molecule⁻¹. From the slope of the van't Hoff plot in the third law analysis, the enthalpy of formation for ClOOCl is calculated, ∆H^◦_f (298K) = 129.9±0.6kJmol⁻¹. The equilibrium constant results from this study suggest that the uncertainties in K_eq recommended in the most recent (year 2015) NASA JPL Data Evaluation can be significantly reduced.