Mobility and toxicity of heavy metals are largely controlled by chemical reactions which take place at the oxide/water interface. Insights in the mechanisms involved in these reactions can be obtained from the recently developed synchrotron-based fluorescence-yield X-ray absorption spectroscopy. The phenomena investigated in the present study are: the precipitation of Cr(HI), its coprecipitation with Fe(III), its sorption onto goethite (αFeOOH), and its oxidation at the surface of a Mn(IV) oxide, birnessite. These examples are used to show how a combination of macroscopic and spectroscopic information provides a mechanistic picture of the mobilization and immobilization pathways of heavy metals in natural systems.