Perchlorate (ClO 4 − ) interferes with uptake of iodide in humans. Emission inventories do not explain observed distributions. Ozone (O 3 ) is implicated in the natural origin of ClO 4 − , and has increased since pre-industrial times. O 3 produces ClO 4 − in vitro from Cl − , and plant tissues contain Cl − and redox reactions. We hypothesize that O 3 exposure may induce plant synthesis of ClO 4 − . We exposed contrasting crop species to environmentally relevant O 3 concentrations. In the absence of O 3 exposure, species exhibited a large range of ClO 4 − accumulation but there was no relationship between leaf ClO 4 − and O 3 , whether expressed as exposure or cumulative flux (dose). Older, senescing leaves accumulated more ClO 4 − than younger leaves. O 3 exposed vegetation is not a source of environmental ClO 4 − . There was evidence of enhanced ClO 4 − content in the soil surface at the highest O 3 exposure, which could be a significant contributor to environmental ClO 4 − .