Abstract <p>Hydronium alunite, (H₃O)Al₃(SO₄)₂(OH)₆, formed during high-pressure acid leaching (HPAL) of nickel laterite ores, poses a critical challenge to sustainable iron recovery due to its high sulfur content, which degrades iron concentrate quality. This study aims to elucidate the thermal decomposition and leaching behavior of hydronium alunite, which remains poorly understood. Using thermogravimetric analysis, differential thermal analysis, mass spectrometry, and X-ray diffraction, thermal decomposition was investigated under argon, air, and hydrogen atmospheres. Complementary leaching experiments with dilute sulfuric acid evaluated the dissolution of decomposition products. Results revealed two decomposition stages: dehydration and desulfurization. Dehydration was atmosphere-independent, while hydrogen markedly accelerated desulfurization. Complete dehydration destabilized the crystal lattice, yielding amorphous intermediates with &gt; 90% Al and S leaching efficiencies. However, high roasting temperatures induced crystallization into γ-Al₂O₃, sharply reducing solubility. Moreover, roasting the HPAL residue at 500&#xa0;°C enabled the selective leaching of Al and S while retaining Fe, confirming that the thermal decomposition and dissolution mechanisms observed in synthetic hydronium alunite apply to the actual HPAL residue. These findings offer critical insight into optimizing thermal treatment for effective sulfur removal and the HPAL residue valorization.</p> Graphical Abstract <p></p>