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 228–301 K 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 ± 25 K/T)} cm³ molecule⁻¹. A second law analysis of the data deviates minimally: K_eq = (2.06 ± 1.27) × 10⁻²⁷ e^{(8546 ± 123 K/T)} cm³ molecule⁻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 (298 K) = 129.9 ± 0.6 kJ mol⁻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.