Mercury (Hg) is a toxic heavy metal that is found associated with fossil fuel deposits and that can be released to the atmosphere during fossil fuel combustion and/or processing. Hg emitted to the atmosphere can be deposited to aquatic and terrestrial ecosystems where it can be methylated by bacteria. Methylmercury is strongly biomagnified in food webs and this leads to toxic levels in high trophic level fish, the consumption of which is a major human exposure pathway. Epiphytic tree lichens have been widely used to investigate the relationship between atmospheric point sources of Hg and regional Hg deposition patterns. An intensive study of Hg concentration and stable isotopic composition of the epiphytic tree lichen Hypogymnia physodes was carried out in the area within 150km of the Athabasca oil sands region (AOSR) industrial developments. Concentrations of Hg were comparable to background values measured in previous studies from remote areas and were far below the values observed near significant atmospheric industrial sources of Hg. Spatial patterns provide no evidence for a significant atmospheric point source of Hg from the oil sands developments, and Hg accumulation actually decreases in lichens within 25km of the northern AOSR development, presumably due to physiological responses of H. physodes to enhanced SO 2 deposition. Stable Hg isotope ratios show an increase in Δ 199Hg and Δ 201Hg within 25km of the AOSR, and we speculate that this is due to a change in the proportion of the various ligands to which Hg is bonded in the lichens, and a resulting change in the isotope fractionation during partial photochemical reduction and loss of Hg from lichen surfaces.