Chlorosulphonic acid is a toxic, highly reactive and corrosive substance that exists in its liquid form at ambient conditions. Its major hazardous potential comes from the clouds of hydrogen chloride and sulphuric acid mist produced whenever this chemical escapes from containment and is exposed to moisture. It decomposes violently and sometimes explosively in the presence of water, liberating heat. On spillage it creates liquid pools that can either boil or evaporate. There are three sources of water available for reaction: free ground water, substrate water and atmospheric moisture. Hydrogen chloride gas or aqueous solution and sulphuric acid liquid are produced by the hydrolysis reaction. This paper describes the dangers involved in cases of accidental releases of chlorosulphonic acid, referring to its properties, toxicity data and mitigation tests. It also reports results of pool behaviour using REACTPOOL [T Kapias, R.F Griffiths, C. Stefanidis, J. Hat. Mat., submitted for publication]. These results indicate that the pool behaviour is governed mainly by the amount of water available for reaction. Surface roughness and wind speed also have a significant effect on the results. A discussion of the results in comparison with those for other water reactive substances is presented in Part III of this series of papers [T. Kapias, R.F Griffiths, J. Hat. Mat., submitted for publication]. The generated cloud will initially contain chlorosulphonic acid, hydrogen chloride and sulphuric acid with numerous processes taking place. Initially, it is usually denser than air. Although chlorosulphonic acid has been involved in major hazard incidents, there are no experimental data relevant to the modelling requirements. Use of REACTPOOL provides insights into the major hazard role of this substance.