The evolution of the atmospheric noble gas composition during the past few decades has hardly been studied because, in contrast to many other atmospheric gases, systematic time-series measurements have not been available. Based on theoretical considerations, the atmospheric noble gas isotope composition is assumed to be stable on time scales of up to about 10⁶ yrs, with the potential exception of anthropogenic changes predicted for the He concentration and the ³He/⁴He ratio. However, experimental assessments of the predicted changes in the atmospheric He isotope composition are controversial. To empirically test these assumptions and predictions, we analysed the noble gas isotope composition in samples of the Cape Grim Air Archive, a well-defined archive of marine boundary layer air in the southern hemisphere. The resulting time series of the ²⁰Ne, ⁴⁰Ar, ⁸⁶Kr and ¹³⁶Xe concentrations and ²⁰Ne/²²Ne and ⁴⁰Ar/³⁶Ar ratios during 1978–2011 demonstrate the stability of the atmospheric Ne, Ar, Kr and Xe composition during this time interval. The He isotope data indicate a decrease in the ³He/⁴He during the same time interval at a mean rate of 0.23–0.30‰ per yr. This result is consistent with most model predictions of the rate of decrease in the atmospheric ³He/⁴He ratio associated with mining and burning of fossil fuels.