Water reactive chemicals are generally aggressive materials that are used widely in the process industries. Common water reactive substances are sulphur trioxide, oleum, titanium tetrachloride, silicon tetrachloride, chlorosulphonic acid, chloroacetyl chloride and phosphorus trichloride. When released to the atmosphere, water reactive materials generally react readily with any free ground water, substrate water and atmospheric water. The exact nature, kinetics and thermodynamics of these reactions govern the subsequent consequences of a release. Consequence modelling and dispersion analysis of water reactive materials is notoriously difficult due to the uncertainties surrounding the reaction with water. Very little experimental data are available on the release behaviour of many water reactive materials. There are often discrepancies and gaps in the data available on the liquid phase hydrolysis reaction; these discrepancies can be as extreme as whether the reaction is exothermic or endothermic. This paper will use a case study to show how the behaviour of water reactive materials has been incorporated into safety and environmental risk assessments, in particular for the purposes of the predictive aspects of COMAH Safety Reports.