Even though Cr(VI) is not used in any step of leather making, the presence of Cr(VI) in leather has become a concern in the leather industry. Free radicals usually play an important role in the formation of Cr(VI) in leather. The effectiveness of antioxidant materials in preventing free radical formation is well known. Tannins as phenolic materials are also known to have antioxidant properties. However, the antioxidant effectiveness shows variations regarding to the type and structure of tannins. In the present study, the vegetable tannins which are commonly used in the leather industry (mimosa, quebracho, sumac, tara, valonea and chestnut) were selected and their antioxidant powers were determined by FRAP (The Ferric Reducing Ability of Plasma) and TEAC/ABTS (Trolox-Equivalent Antioxidant Capacity/2,2'-azinobis-(3-ethylbenzothiazoline-6-sulfonic acid)) methods. Furthermore, these vegetable tannins were used in leather processing and their effect on prevention of Cr(VI) formation was examined. The results show that gallotannins within the hydrolysable tannins have taken the first places in ranking with superior antioxidant activities and, compatible with their antioxidant powers they were found to be the most effective tannin types at preventing Cr(VI) formation.