Interacting chemical systems can be analyzed using the structures and lifetimes of characteristics modes of variation about a quasi-steady state. For the atmospheric CH4--CO--OH system, characteristic modes of concentration variation involve predominantly a single species, but their lifetimes differ from the dominant species turnover times. Here a species turnover time is the species amount divided by its removal rate, whereas the characteristic mode lifetime is the amplitude of a perturbation pattern divided by its rate of change and includes feedback effects. Characteristic mode analysis is used here to consider variations in both concentrations and isotopic ratios for such systems. New 'isotopic modes' are identified which are only weakly affected by feedbacks and have lifetimes very close to species turnover times. This raises the possibility that analysis of variations in both concentration and isotopic ratios may provide a unique tool for resolving turnover times and chemical feedbacks in atmospheric chemistry. ¿ 1999 American Geophysical Union