Antimony (Sb) is a critical metal and emerging hazardous pollutant, chemically akin to arsenic (As) and widely used as antimonial lead in Pb–Sb–(Sn) alloys. Despite its volatility, ~45–50% of Sb input to municipal solid waste incinerators remains in bottom ash (IBA). Sb leaching, particularly during mineral carbonation, compromises IBA reuse. We report the systematic occurrence of Pb–(Sb)–(Sn) particles in IBA fines (< 2 mm), spanning a continuum of alteration states. When oxidized, these particles display distinctive yellow to brown-reddish colours and were found in all four samples analysed from the ASHES academia–industry innovation program. Crystal chemistry and formation pathways were resolved at the particle scale using SEM-EDS, XRPD, and TEM-EDXS-EELS, confirming the presence of Sb(0), Sb(III), and Sb(V). Carbonation behaviour was assessed by blending Ca(OH)₂ with crushed Pb–(Sb)–(Sn) particles. Carbonation enhances the dissolution of Pb–(Sb)–(Sn) oxides, with Pb reprecipitating as carbonates with calcite, while Sb remains in solution, which explains the inverse Pb–Sb leaching behaviour widely reported. The strong Pb–Sb association has treatment implications, suggesting that density separation of IBA fines prior to carbonation could mitigate Sb release. We propose that this phase is the main source of Sb leaching under IBA weathering conditions, as metallic Pb–Sb assemblages are far less volatile than Sb species in plastics. During incineration, they remain on the grate and partially crystallize, whereas Sb from plastics is more likely to volatilize and be captured in fly ash. Sb partitioning during incineration is thus strongly source-dependent, reflecting the distinct thermal behaviours of its main industrial forms.