Our group studies the speciation (different chemical species or molecules) of trace elements such as arsenic, antimony or mercury in the environment (air, soil, sediments, water and plants). Indeed, total concentrations of these elements only give a limited view of their biogeochemical cycle since different chemical species possess different mobilities, toxicities and even physico-chemical properties (e.g. inorganic mercury vs. methylmercury). Our focus is mainly on the biomethylation and biovolatilisation of arsenic, antimony and mercury in soils since very little is known about these two intertwined biological mechanisms although they play an important role in the biogeochemical cycle of these elements. To do this, we develop and adapt state-of-the-art extraction and analytical methods such as HPLC-ICP-MS, HPLC-ESI-MS and stable isotope approaches. We then use these methods in soil incubations and in the field to determine the drivers of biomethylation and biovolatilisation and their relative importance toward the global biogeochemical cycle of trace elements.