Antimony (Sb) derived from historic mining waste from the Hillgrove gold/Sb mine has dispersed over 300km through the Macleay River Catchment of south-east Australia. Stibnite and Sb-rich ore form the bulk of the dispersed material, with minor arsenopyrite. The resulting co-contamination of the Macleay River has been well-characterised, but the processes underlying the sequestration of As and Sb in the catchment are unknown. This study aimed to characterise metalloid mobilisation and sequestration processes whilst also identifying diferences in the geochemisty of the two elements in this system. Sediments were collected from 13 sites spanning the length of the waste dispersal pathway in the river system. Sediment elemental content was determined with aqua regia digestion and inductively coupled plasma optical emission spectrometry (ICP-OES). Elemental composition of individual grains from high concentration sites was undertaken using electron microprobe. Extracts targeting the water soluble (H₂O), easily exchangeable (0.5M NaHCO₃), and amorphous-associated (0.5M acid ammonium oxalate) metalloid content of sediments were analysed using ICP-mass spectrometry and atomic fluorescence spectrometry (AFS). Iron (Fe) was significantly positively correlated with total As and Sb in the lower Macleay River, while in the more heavily contaminated upper reaches only As was correlated with Fe. Electron microprobe analysis of highly contaminated sediments identified grains of high purity Sb₂O₃. Arsenic was identified at concentrations up to 2.5 % of total grain weight, but only in association with Fe oxide grains. The three-step extraction showed that a similar proportion of total Sb (59 – 88 %) and As (59 – 82 %) remained in the residual fraction of the upper catchment sediment, whilst in the sediments of the lower river residual As (51 – 71 %) was significantly greater than Sb (27 – 53 %). Antimony in sediment was more water soluble than As throughout the catchment (0.6 – 2.2 % total Sb compared with 0.4 – 1.4 % of total As), but 0.5M NaHCO₃ extractable As was greater than Sb (by 1.9 – 5.3 times). Metalloid association with amorphous phases was significant and positively correlated with distance from the upper catchment source, being up to 29 – 49 % of total As and 22 – 64 % of Sb in lower catchment sediments. Antimony associated with amorphous phases was significantly lower at two sites often subject to anoxic conditions (22 – 28 % of total Sb) compared with nearby aerated sediments (47 – 63 %). This variability was not observed for As. The results suggest that As has been more completely dissolved from its mineral sources than Sb, and been significantly incorporated into Fe hydroxyoxides throughout the dispersal pathway. In contrast Sb remains in high purity forms in the upper Macleay River, and becomes increasingly associated with amorphous phases in the lower reaches. The sediment associations of Sb were more variable than those of As when comparing sites with different redox conditions. This study provides critical insights into metalloid sequestration processes under a range of environmental conditions which is vital to fully quantify the risks posed by As and Sb in contaminated river catchments.