Functional groups at hydrous oxide surfaces can form inner-sphere complexes with transition-metal ions. Such specific interactions may catalyze redox processes at surface-bound metal centers in various ways. Three case studies, of interest in the geochemistry of natural waters, are presented with an attempt to provide some mechanistic insight into the catalysis of redox reactions at hydrous oxide surfaces: (1) the oxygenation of adsorbed vanadyl; (2) the light-induced and (3) the Fe(II)-catalyzed reductive dissolution of Fe(III)-(hydr)oxides. It is shown that the rate of vanadyl oxygenation is accelerated by surface MO- groups; a similar effect is observed in solution due to coordination by OH-. Fe(III)-(hydr)oxides to which potentially reducing ligands are specifically adsorbed (e.g., hematite in the presence of oxalate) become subject to light-induced reductive dissolution. A catalytic dissolution of hematite by Fe(II) and oxalate is interpreted in terms of an electron transfer through the bridging oxalate ligand, FE(III)-oxalate-Fe(II).