Selective extractions are widely used in sediment, soil and sludge samples to assess trace element carrier phases, mobility and potential bioaccessibility. Commonly used selective parallel extractions were applied to natural and isotopically-labelled bulk sediments from the Gironde fluvial-estuarine system and the Rhône River (France) to determine the solid phase fractionation of antimony (Sb), a priority contaminant in EU and US regulations and an important radionuclide in decay series occurring in the environment after nuclear power plant accidents. Antimony fractions obtained from several, parallel selective extraction solutions targeting Fe/Mn oxides (i.e., hydroxylamine-, oxalate- and ascorbate-based, herein assigned as fractions from “F2” solutions) and acid-soluble operationally defined phases (HCl- and HNO3-based, herein assigned as fractions from “F4” solutions) were compared (i) between each other's, and (ii) with those of other trace elements (Co, Cu, Ni, Pb, Zn, Th and U) in the same extractions. The solid fractionation of inherited Sb and spiked Sb was studied by applying a complete set of parallel selective extractions to isotopically-labelled sediments of the Gironde Estuary. Results suggest protocol-related and sediment-dependent anomalies in Sb selective extractions, compared to results expected from the operationally-defined extraction scheme and obtained for other trace elements. In fact, Sb fractions extracted with oxalate- and ascorbate-based solutions were greater than acid-soluble fractions (1M HCl and 1M HNO3) in the Garonne/Gironde fluvial estuarine sediments. A similar anomaly occurred for oxalate-based extractions of Sb in Rhône River sediments. These observations suggest that reducing conditions and the presence of strongly complexing organic ligands in the environment may mobilise respectively 2-fold and ~5-fold more Sb from natural bulk sediment and from sediments spiked with isotopically-labelled Sb than the acid-soluble fraction, usually used to assess the reactive, potentially bioaccessible fractions. The underestimation of the reactive, potentially bioaccessible fractions of Sb may bias the environmental interpretation of Sb solid fractionation and should be taken into account when assessing stable Sb cycles and Sb radionuclide dispersion scenarios in continent-ocean transition systems.