Iron monosulfide-rich sediments are commonly found in drains in acid sulfate soil (ASS) areas in eastern Australia. Sulfides represent an important sink for contaminants (including acidity and metals) and play an important role in improving water quality. However, these sediments are prone to oxidation during drain cleaning or drought, releasing acidity and metals into the drainage system, and are a potential environmental hazard in ASS landscapes. Chemical changes during oxidation of iron monosulfide-rich sediments have been studied in an incubation experiment. The effect of lime and mill mud/ash (an alkaline sugar mill byproduct) as neutralising agents has been examined. Oxidation of untreated sediments resulted in decreased pH, the production of acidity, and release of SO₄ and metals, including iron, aluminium, and manganese. The large concentration and highly reactive nature of iron monosulfides suggest that the chemical changes observed are mostly a result of iron monosulfide oxidation, although the contribution of pyrite oxidation cannot be totally excluded. Lime appears to be an effective neutralising agent by maintaining pH and low acidity concentrations throughout the incubation experiment. The mill mud/ash initially prevented oxidation but its effectiveness was limited. These findings are important in terms of drain management in ASS environments.