Oxalate (C2O42-) acts synergistically on the dissolution of goethite (alpha-FeOOH) in the presence of siderophores that are secreted by plants and microorganisms to sequester iron. We report here the first in situ molecular-scale observations of synergistic ligand-promoted dissolution processes. We show that there are conditions under which oxalate promotes goethite dissolution, but dissolved Fe(III) concentrations do not increase because Fe(III)-oxalate complexes readsorb to the mineral surface. We demonstrate that these readsorbed Fe(III)-oxalate complexes are highly mobile, extremely reactive in the presence of uncomplexed siderophores, and responsible for the synergistic effects on the dissolution of goethite.