Chlorine gas evolution was investigated by bubbling O 2 gas into molten CaCl 2 at temperatures higher than 1173 K, although a thermodynamic analysis shows almost no progress of the reaction 2CaCl 2 + O 2 ) 2CaO + Cl 2 . To break the thermodynamic limitation, chlorine gas slightly evolved was quickly removed from the reactor by flowing O 2 . From the data obtained at the early stage of reaction, the rate equation for Cl 2 evolution was determined to be d[Cl 2 ]/dt ) k(P O 2 ) n / (1 + k′ [CaO]), where 0.7 < n <0.9, with an activation energy of 157.6 kJ/mol. Upon prolonged reaction, the rate of Cl 2 evolution rapidly decreased because of the progress of the reverse reaction between Cl 2 and CaO generated during the reaction. Through the addition of silica powder into molten CaCl 2 , the rapid decrease in the Cl 2 evolution rate on prolonged reaction was significantly moderated: about 60% of the Cl ions contained in the CaCl 2 were recovered as Cl 2 gas. This is due to the reaction of SiO 2 with CaO to form calcium silicate, which is less reactive with Cl 2 gas.