A vital design parameter for any in situ chemical oxidation system using permanganate (MnO4-) is the natural oxidant demand (NOD), a concept that represents the consumption of MnO4- by the naturally present reduced species in the aquifer solids. The data suggest that the NOD of the aquifer material from Canadian Forces Base Borden used in our study is controlled by a fast or instantaneous reaction captured by the column experiments, and a slower reaction as demonstrated by both column and batch test data. These two reaction rates may be the result of the reaction of MnO4- with at least two different reduced species exhibiting widely different rates of permanganate consumption (fast rate >7 g of MnO4- as KMnO4/kg/day and slow rate of approximately 0.005 g/kg/day), or a physically/chemically rate-limited single species. The slow NOD reaction prevented fulfillment of the ultimate NOD during the days- to months-long batch experiments and allowed significant early MnO4- breakthrough (>98%) during transport in the column experiments. A large fraction of the organic carbon resisted oxidation over the 21-week duration of the batch experiments. This result demonstrates that NOD estimated from total organic carbon measurements can significantly overpredict the NOD value required in the design of an in situ chemical oxidation application.