A recent paper by Tans [1997] has drawn attention to the isotopic disequilibrium that inevitably prevails when atmospheric methane is not in steady state with its sources, noting in particular the very slow adjustment of the isotopic signature &dgr;13C toward its steady state. Our aim in this paper is to clarify the nature of disequilibrium effects on &dgr;13C(CH4) and to assess their likely magnitudes in the global atmosphere over recent decades. We use a simple model simulation incorporating a plausible scenario of the global methane source history over 1700--2010, which includes an unchanged source since 1990. The simulation of both mixing ratio and &dgr;13C compare favorably with the secular features of a 10-year data set (1988--1998) from Baring Head, New Zealand, and of a 17-year data set (1978--1995) in air archived from Cape Grim, Australia. This corroborates a recent analysis of those data sets and their compatibility with stabilized sources. We show that the slow adjustment of &dgr;13C toward steady state arises from the effect of isotope fractionation on the cancellation of contributing terms to &dgr;13C. We explore the implications of disequilibrium for the usual practice of relating &dgr;13C values in the atmosphere to those in the aggregate source through a shift induced by fractionation and quantify the flaws in this practice. Finally, we examine the sensitivity of the atmospheric secular response, in both mixing ratio and &dgr;13C, to sustained changes in source and sink and show that &dgr;13C is a potentially powerful diagnostic of such changes. ¿ 2000 American Geophysical Union