A recent paper by Tans  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 δ13C toward its steady state. Our aim in this paper is to clarify the nature of disequilibrium effects on δ13C(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 δ13C 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 δ13C toward steady state arises from the effect of isotope fractionation on the cancellation of contributing terms to δ13C. We explore the implications of disequilibrium for the usual practice of relating δ13C 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 δ13C, to sustained changes in source and sink and show that δ13C is a potentially powerful diagnostic of such changes.