Sulphur Mustard, or "Mustard Gas" is in fact an oily liquid which was used as a chemical weapon primarily for its vesicant action which necessitates whole body protection. It is also now recognised as a carcinogenic agent upon chronic exposure. Soil contaminated with Sulphur Mustard continues to present both acute and chronic human health risks and risks to groundwater, surface water and the wider ecology at a number of sites globally and, in some cases, has done for many decades. This is at odds with the simple aqueous chemistry of the compound which would suggest that it should be short lived in the environment, especially in the presence of water. A number of studies have examined the possible factors for this longevity and, though the causes are generally assumed to be understood, the precise reasons have not yet been definitively determined and the evidence in support of the existing theories is at best circumstantial. At present, the prevailing view is that Sulphur Mustard is somehow protected by oligomeric or polymeric sulphonium species produced during incomplete hydrolysis reactions. The following review discusses the pertinent degradation mechanisms in the environment; hydrolysis and thermal degradation and the reasons put forward for the longevity of Sulphur Mustard in the literature. Other factors, such as the role of polymeric species in Sulphur Mustard droplets in modifying the mobility of the agent are also examined. Ultimately, without a thorough understanding of the abiotic fate of the Sulphur Mustard, uncertainties will remain in the application of risk assessment and remediation strategies to such sites, potentially compromising the validity or effectiveness of such actions.