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Phosphate and other oxo-anions have been shown to stimulate the rate of iron loading into ferritin (J. Polanams, A.D. Ray, R.K. Watt, Inorg. Chem. 44 (2005) 3203-3209). This study was undertaken to determine if accelerated iron loading was a specific effect for phosphate and closely associated oxo-anions or if it was a general anion effect. Controls were also performed with mono-valent cations to determine the effect of these cations on iron loading into ferritin. Cations were shown to slow the rate of iron loading into ferritin. Fluoride and iodide were shown to slow the iron loading process of ferritin. Sulfate was also shown to slow iron loading into ferritin to a more significant extent than the cations or halides tested. The trigonal planar oxo-anions, carbonate and nitrate, did not inhibit or stimulate iron loading. We conclude that the increased rate of iron loading into ferritin is specific to phosphate and other closely associated tetrahedral oxo-anion analogs, that the effect is driven by the insolubility of the iron and anion complex, and that in general, cations and anions slow the rate of iron loading into ferritin.