Recent measurements of the apparent kinetic isotope effect (KIE) of the methane (CH4) atmospheric sink in the extratropical Southern Hemisphere (ETSH) have shown the apparent KIE to be larger in magnitude than expected if the sink were the hydroxyl radical (OH•) alone. We present results from simulations using the U.K. Met Office's Unified Model (UM) to evaluate whether atomic chlorine (Cl•) in the marine boundary layer (MBL) could give this effect. We modify the UM to include sources of 12CH4 and 13CH4, soil and stratospheric sinks, and a tropospheric OH• sink. Also included is a Cl• sink in the MBL with a large seasonal cycle and a constant mean value (Cl• mean) in latitude. We show that analysis of the simulated seasonal cycles in CH4 mixing ratio and δ 13C give an accurate estimate of the OH• KIE at ETSH midlatitudes. The apparent KIE of the combined OH• and Cl• sink increases in magnitude as Cl• mean increases. The experimentally measured values of apparent KIE in the ETSH midlatitudes of -15? in 1994--1996 and -7? in 1998--2000 are attained with MBL Cl• mean values of 28 W 103 atoms cm-3 and 9W103 atoms cm-3, respectively (although we consider the latter to be a lower bound). We suggest that 18W103 atoms cm-3 is a reasonable midrange estimate of Cl• mean in the MBL. This value results in a Cl• sink strength of 25 Tg y-1 (range 13--37 Tg y-1) and an enrichment in δ 13C of atmospheric CH4 by 2.6? (range 1.4--3.8?). This sink strength is significant but has not yet been included in global CH4 budgets.