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Caesium-137 (¹³⁷Cs) and iodine-131 (¹³¹I) are radionuclides of particular concern during nuclear accidents, because they are emitted in large amounts and are of significant health impact. ¹³⁷Cs and ¹³¹I attach to the ambient accumulation-mode (AM) aerosols and share their fate as the aerosols are removed from the atmosphere by scavenging within clouds, precipitation and dry deposition. Here, we estimate their removal times from the atmosphere using a unique high-precision global measurement data set collected over several months after the accident at the Fukushima Dai-ichi nuclear power plant in March 2011. The noble gas xenon-133 (¹³³Xe), also released during the accident, served as a passive tracer of air mass transport for determining the removal times of ¹³⁷Cs and ¹³¹I via the decrease in the measured ratios ¹³⁷Cs/¹³³Xe and ¹³¹I/¹³³Xe over time. After correction for radioactive decay, the ¹³⁷Cs/¹³³Xe ratios reflect the removal of aerosols by wet and dry deposition, whereas the ¹³¹I/¹³³Xe ratios are also influenced by aerosol production from gaseous ¹³¹I. We find removal times for ¹³⁷Cs of 10.0–13.9 days and for ¹³¹I of 17.1–24.2 days during April and May 2011. The removal time of ¹³¹I is longer due to the aerosol production from gaseous ¹³¹I, thus the removal time for ¹³⁷Cs serves as a better estimate for aerosol lifetime. The removal time of ¹³¹I is of interest for semi-volatile species. We discuss possible caveats (e.g. late emissions, resuspension) that can affect the results, and compare the ¹³⁷Cs removal times with observation-based and modeled aerosol lifetimes. Our ¹³⁷Cs removal time of 10.0–13.9 days should be representative of a "background" AM aerosol well mixed in the extratropical Northern Hemisphere troposphere. It is expected that the lifetime of this vertically mixed background aerosol is longer than the lifetime of fresh AM aerosols directly emitted from surface sources. However, the substantial difference to the mean lifetimes of AM aerosols obtained from aerosol models, typically in the range of 3–7 days, warrants further research on the cause of this discrepancy. Too short modeled AM aerosol lifetimes would have serious implications for air quality and climate model predictions.