Abstract: The chlorine and oxygen content in anode gas has been determined by gsa absorption method. The result shows that the oxygen evolution rale changes with pHs (the pHvalue of anode chamber electrolyte): when pHs<2, the rale of oxygen evolution increascs rapidly with increasing pHs; between 2-4.2, the rate of it changed slowly, and, when pHs>4.2 it increases again quite rapidly. The rule stated above are independent of the density of current and the constitution of the oxide film of the electrodes within the current density range 150-550mA/cm².