Much of Western Europe has inherited soil contaminated with heavy metals from past mining, mineral processing and industrial activities. When performing risk assessment for eco-toxicological, human health or ground water vulnerability studies the major issue is not the total concentration of the heavy metal but its labile fraction i.e. the proportion of the metal transferable to an aqueous phase in ionic form. This may vary widely as a function of the solid forms in which the metal is hosted, the pH and redox conditions in pore waters that control the lability, or the occurrence of other species within the aqueous phase which may provide competitive sorption or preferential transport modes. Given the toxicity of a number of heavy metals even at low contents, accurate methods are required to properly monitor them in the environment and gain further insight in their behaviour. Simple extraction schemes are the most common procedure to assess natural availability of heavy metals in different scenarios. However, it has been demonstrated that these are unable to measure the true proportion of metal that may be exchangeable and contribute to lability, which may lead to unreliable risk assessments. This proposal seeks to accurately study the lability of heavy metals in several polluted scenarios by applying one of the most advanced methodologies for this purpose, namely ‘isotope dilution’ (ID). This technique reflects the pool of reactive metal in the soil and can be used to model solid-solution equilibria and the fixation of metal ions into less available forms. ID method has been successfully implemented for Cd, Zn and As, while a method for environmentally significant elements such as Fe, Sb and Se -particularly susceptible to redox conditions- remains to be developed. Data will be combined with soil pore water analyses to geochemically model scenarios and determine the role of changing redox conditions in the releases of heavy metals to the environment.