Adjusting the concentration of Fe(II) or Fe(III) in the polyol reduction of silver nitrate allows for the production of either nanocubes or nanowires. Reduction by ethylene glycol maintained the iron ions in the reduced Fe(II) form, which in turn reacted with and removed adsorbed atomic oxygen from the surface of silver nanoparticles. Relatively high concentrations (2.2 mu M) of iron ions facilitated the growth of multiply twinned seeds to form nanowires by removing oxygen from the surface of twinned seeds and preventing heir dissolution by oxidative etching. Lower concentrations (< 0.44 mu M) of iron ions only partially removed oxygen from the surface, resulting in selective etching of twinned seeds and accelerated formation of single-crystal nanocubes. In each case, the addition of chloride prevented rapid aggregation of seeds and facilitated their growth into discrete nanostructures.