The effect of Fe(II) on the performance of barite scale inhibitors was tested using an improved anoxic testing apparatus. Inhibitors were tested with from 1 to 50 mg/L Fe(II) at 70oC and near neutral pH conditions. Most scale inhibitors show good Fe(II) tolerance at experimental conditions, while some phosphonates based scale inhibitors were significantly impaired by Fe(II). The formation of insoluble precipitates between Fe(II) and phosphonate is very likely the reason behind this detrimental effect. Fe(III) can significantly impair the performance of all scale inhibitors even at extremely low concentrations. However, the mechanism of this detrimental effect has not been studied. In this research, an analytical ultracentrifuge was utilized to separate ferric hydroxide nanoparticles from the aqueous phase. Scale inhibitor concentration in the aqueous and particle phases were measured and compared with barite induction time data. The mechanism of Fe(III) effect on scale inhibitor was experimentally shown a result of adsorption of scale inhibitor onto ferric hydroxide nanoparticles in solution. If inhibitors are added in excess of the adsorption ability of the ferric hydroxide particles, the remaining scale inhibitors in the aqueous phase can still provide inhibition. EDTA and citric acid, two of the most common organic chelating agents used in oilfield, were tested for their ability to reverse the detrimental effect of Fe(III) on scale despite the fact the EDTA is a much stronger chelating agent. The mechanistic difference between EDTA and citrate is discussed.