Chlorine gas evolution was investigated by bubbling O-2 gas into molten CaCl2 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(Po-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 CaCl2, the rapid decrease in the Cl-2 evolution rate on prolonged reaction was significantly moderated: about 60% of the Cl ions contained in the CaCl2 were recovered as Cl-2 gas. This is due to the reaction of SiO2 with CaO to form calcium silicate, which is less reactive with Cl-2 gas.