The equilibrium constants for the formation of ClOO from Cl and O-2 are experimentally measured at 212-245 K using cavity ring-down spectroscopy. A van't Hoff plot analysis yields DeltaH(r) = 4.8 +/- 1.5 kcal mol(-1). The Cl-OO bond dissociation energy is determined to be 4.69 +/- 0.10 kcal mol(-1) from the present and previously reported temperature dependences of the equilibrium constant by the third-law analysis utilizing our recent accurate rotational spectroscopic data. High level ab initio calculations using MRSDCI+Q with the complete basis set extrapolation are also performed. The present ab initio calculations yield the Cl-OO bond dissociation energy to be 4.56 kcal mol(-1).