Mercury is easily transformed from the elemental state or numerous other mercury compounds to its gaseous form. The gaseous mercury species remain in the atmosphere for sufficient time to be carried long distances. In many cases, some of these differences produce a bias in experimental research, especially for the understanding of the soil-plant-atmosphere system in polluted sites. The main controversy concerns the preferential uptake route in plants and whether the process is reversible or not. The aims of the work described here were to acquire a better knowledge of the transfer routes in the soil-plant-atmosphere system and to ascertain whether plant mercury uptake is reversible. Factors such as the presence of mercury in the atmosphere and irrigation water were evaluated using two common species in Castilla-La Mancha: vines (Vitis vinifera, L.) and olive trees (Olea europea, L.). The results provide evidence of three important aspects: plant mercury uptake via roots does occur but it seems minor; plant mercury uptake from the atmosphere involves a continuous exchange at the leaf-atmosphere interface; and plant mercury bioaccumulation seems to be a reversible process. These findings have implications for the use of plants as biomonitors by restricting the exposure times, and in phytoremediation, particularly for polluted soils.