Recent investigations have reported a widespread occurrence of chlorate (ClO 3 − ) and perchlorate (ClO 4 − ) throughout the solar system, including terrestrial arid environments. ClO 3 − and ClO 4 − are deposited/ accumulated at an approximate equal molar ratio, with some exceptions, such as the Antarctica Dry Valley soils (MDV) and perhaps Martian surface material, where ClO 4 − is the dominate ClO x − species. All known ClO 4 − production mechanisms produce molar ratios of ClO 3 − /ClO 4 − equal to or much greater than 1, suggesting that reduced ratios may be due to post-depositional mechanism(s). The objective of this study was to investigate potential iron-mediated abiotic reduction of ClO 3 − , similar to transformation mechanisms reported for nitrate (NO 3 − ) by Fe(II) minerals. Three types of Fe(II)-containing minerals, wustite (FeO), siderite (FeCO 3 ), and sulfate green rust (GR SO4 2− ), were investigated in completely mixed batch reactors as potential ClO 3 − reductants at a range of pH (4− 9) and iron mineral concentrations (1−10 g/L). ClO 3 − was stoichiometrically reduced to chloride (Cl − ) by wustite, siderite, and green rust, but no transformation occurred by dissolved Fe(II). Wustite and green rust reduced NO 3 − but not by siderite. When both NO 3 − and ClO 3 − are reduced simultaneously, ClO 3 − is reduced preferentially to NO 3 − , although the effect is somewhat concentration-dependent. An increased background salt concentration (NaCl) increased ClO 3 − reduction but decreased NO 3 − . The stability of ClO 3 − and subsequent impacts on the ratio of ClO 3 − /ClO 4 − in the environment have implications for understanding the cycling of oxyanions and stability of iron minerals, and related to this, the ratio of ClO 4 − and ClO 3 − may be an indicator of the past availability of free water. On Mars, these reactions may help to explain the unusually high concentrations of ClO 4 − compared to ClO 3 − and NO 3 − .