In context with the frequent observations of excessive iron (Fe) storage in captive black rhinoceroses (Diceros bicornis), it has been suggested that both an excessive dietary Fe content and a lack of dietary Fe-chelating substances, such as tannins, is the underlying cause. Therefore, studies on the effects of tannin supplementation to captive diet are warranted. Six captive rhinoceroses were fed their normal zoo diet (N), and a similar diet supplemented with either tannic acid (T, hydrolysable tannin) or quebracho (Q, condensed tannins), and the total antioxidant capacity (TAC) was measured as mmol Trolox equivalents per kg fresh faeces. The TAC values on diets N (1.24 +/- 0.39 mmol/kg fresh faeces) and T (1.34 +/- 0.33 mmol/kg fresh faeces) were similar, but significantly higher on diet Q (2.32 +/- 0.61 mmol/kg fresh faeces). In contrast to expectations, faecal TAC increased with increasing faecal Fe, possibly as a result of the fact that the faecal Fe content was positively correlated to the proportion of concentrate feeds in the diet, which also contain antioxidants, such as vitamin E, in addition to Fe. Increased antioxidant status caused by the use of tannin substances could have a beneficial effect on animal health, but if tannins should be incorporated in designed diets, other tannin sources, such as grape pomace should be tested.