Globally, mining activities have been responsible for the contamination of soils, surface water and groundwater. Following mine closure, a key issue is the management of leachate from waste rock accumulated during the lifetime of the mine. At Ranger Uranium Mine in northern Australia, magnesium sulfate (MgSO4) leaching from waste rock has been identified as a potentially significant surface and groundwater contaminant which may have adverse affects on catchment biota. The primary objective of this study was to determine the effect of elevated levels of MgSO4 on two riparian trees; Melaleuca viridiflora and Alphitonia excelsa. We found that tolerance to MgSO4 was species-specific. M. viridiflora was tolerant to high concentrations of MgSO4 (15,300 mg l-1), with foliar concentrations of ions suggesting plants regulate uptake. In contrast, A. excelsa was sensitive to elevated concentrations of MgSO4 (960 mg l-1), exhibiting reduced plant vigour and growth. This information improves our understanding of the toxicity of MgSO4 as a mine contaminant and highlights the need for rehabililitation planning to mitigate impacts on some tree species of this region.