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The interaction of toxic oxyanions with organic matter can affect their stability and binding mechanisms in soils. The adsorption/desorption properties of arsenic (AsV)/antimony (SbV) oxyanions were tested with two types of soil with a variable content of natural organic matter (NOM). Sample JEZ (I) represented a locality near a brown-coal power plant, and therefore formerly exposed to large amounts of atmospheric pollution, while sample LIZ (II) originated in an uncontaminated region without anthropogenic impact. Soil samples were investigated from the organic O and mineral B horizons. Different As/Sb fixations were indicated by their different adsorption affinity to O and B horizons. Arsenic as AsV was strongly adsorbed in B horizons, while antimony as SbV was predominantly bound in O horizons. The sorption followed the Langmuir model with maximum adsorption capacities of 1.4 × 10−2 mmol g−1 for As and 1.0 × 10−2 mmol g−1 for Sb adsorptions. The release of As and Sb to distilled water and 0.1 M KCl illustrated the significantly larger stability of As and Sb oxyanions in ionic solutions, where available ions prevented the aggregation of organic matter, thus maintaining the balanced distribution of surface charge. Distilled water with a pH of about 6 supported a negative surface charge density and enhanced the release of As/Sb oxyanions.