The absorption of food iron can be greatly influenced by other constituents in the diet such as ascorbic acid (Vitamin C) and phenolics. Ascorbic acid increases the bioavailability of iron by converting Fe³ to Fe² , while phenolics can reduce the bioavailability of iron by binding to it. Phenolics (e.g., tannins), however, are often not included in diets of captive animals because they 1) can act as digestibility reducers by binding to dietary protein, and 2) require the feeding of fresh leaf material which can be hard to obtain on a regular basis. Conversely, ascorbic acid levels can be excessive from fresh fruit (e.g., oranges) and green vegetables that are often fed. Captive folivorous and frugivorous animals, in particular, can be affected by excessive uptake of dietary iron leading to clinical/pathological iron storage disease; excess ascorbic acid and/or a lack of dietary tannins have both been suggested as contributing to the incidence of these health conditions. Experiments reported herein determined the effects of tannic acid, catechin, chlorogenic acid, and gallic acid and a variety of plant extracts on the in vitro solubilization of iron. Binding curves used a stock solution of ferric ammonium sulfate (FAS) to which the pH was varied. Putative plant tannin/polyphenolic extracts were prepared from red seedless Thompson grapes, Black Riber grapes, blueberries, blackberries, walnut hulls, and grape seed extract. Binding was measured colorimetrically as absorbance at λ max. Most pure compounds tested and the putative tannin/polyphenolic sources bound iron, except several samples of red seedless grapes and blueberries. Grape seed extract, sold commercially as an antioxidant dietary supplement, was especially effective at binding iron. Iron binding was effectively quenched in some samples when ascorbic acid concentrations were elevated. Understanding the interactions among iron, phenolics, and ascorbic acid in vitro will assist in modeling and developing optimal diets for managed feeding programs and overall improved health in captive zoo animals.