Inclusion of kinetic isotope effects (KIEs) of methane (CH4) sinks (gaseous OH and Cl, and soil microbes) has a significant effect on modeled distributions of Δ13C of atmospheric CH4. For a given scenario of surface sources and corresponding Δ13C values of individual CH4 sources, the KIE due to soil uptake enriched Δ13C by 1.18? in the model's northern hemisphere (NH) (40 ¿N) and 1.16? in the southern hemisphere (SH) (40 ¿S) under steady state conditions in January. The KIE due to CH4 oxidation by stratospheric Cl radicals further enriched these Δ13C values at the surface by 0.99? and 1.03?, respectively. In the vertical direction, during January at 50 ¿N, inclusion of a KIE due to Cl enriched Δ13C at 18 km by 0.95? compared to the corresponding surface value, whereas the enrichment was only 0.31? when this KIE was omitted. These results suggest that modeling of Δ13C distributions should include KIEs due to CH4 oxidation by soil and stratospheric chlorine radicals. It is shown that possible oxidation of CH4 in marine boundary layer by Cl radicals can significantly enrich Δ13C. However, if a recent theoretical value for the KIE of the Cl and CH4 reaction is correct, then the impact of this reaction is less than the figures quoted above. In the model, monthly variations in OH concentration and interhemispheric exchange transport cannot reproduce the observed seasonal amplitude variation of Δ13C in either the NH or SH. It is argued that seasonal variations in individual CH4 fluxes are primarily responsible for this discrepancy. We show that increasing Cl radical concentrations due to continued release of anthropogenic chlorocarbons enrich the Δ13C values. The effects of an increase in tropospheric OH concentration due to stratospheric ozone depletion and a cooling of the troposphere due to the eruption of Mt. Pinatubo, with a lowering of water vapor concentration and reduction in isoprene emissions on Δ13C and surface CH4 mixing ratios are investigated. Other model simulations with adjusted surface CH4 fluxes have been performed to study the postulated explanations for recent changes in CH4 surface mixing ratios and Δ13C values. A modified version of the Oslo two-dimensional global tropospheric photochemical model was used for all simulations. ¿ American Geophysical Union 1996