Recently, isotopically extremely light CO (Δ13C=-32 to -43?) was found in whole air samples from the high southern-latitude lower stratosphere (Part I). To investigate the cause of these unprecedented observations, we simulate in a photochemical model the typical history of air masses at polar latitudes in the lowermost stratosphere over austral winter and spring. The model results show that the observations may be explained by the very efficient isotope fractionation in the reaction R1:CH4+Cl→HCl+CH3. Extremely high levels of Cl atom concentrations prevail in the polar lower stratosphere in austral spring for a period of about 10 days after most ozone is lost. Therefore, R1 becomes very efficient, leading to a very high rate of buildup of HCl and an unusually high isotopic CO fractionation. ¿ American Geophysical Union 1996