Chlorine-36 (half life 3.01 9 10 5 year), a beta emitter, is produced naturally but its presence has been enhanced by atmospheric weapons testing and other nuclear activities. Iodine-129 has a half life of 1.57 9 10 7 years and is also produced by nuclear activities, in particular fuel reprocessing. Many elements have a long biological half-life in lichens, which were thus investigated so as to assess their suitability for 36 Cl and 129 I monitoring. Lichens sampled between 1998 and 2008 were analysed for total chlorine, and selected samples were processed for 36 Cl measurement using Accelerator Mass Spectrometry (AMS); 129 I was analyzed by gamma spectrometry. Different aspects are discussed: long-term storage in lichens versus environmental mobility, levels in samples collected near a reprocessing facility, and potential for spatial and temporal monitoring.