phosphorus trichloride and oxychloride are aggressive materials, widely used in the process industries. On escape to the atmosphere they create toxic clouds that may cause serious damage to people and to the environment, When spilled onto the ground they create liquid pools that can boil, evaporate or even solidify. The main feature of the pool behaviour is the exothermic reaction of these chemicals with water, which is complicated and depends heavily on the amount of water available for reaction, and as result of which the pool has changing composition and properties. The purpose of this paper is to describe the dangers involved in cases of accidental releases of phosphorus trichloride and oxychloride, to report their properties, referring to toxicity data and major accidents. The spill behaviour of phosphorus trichloride and oxychloride has been incorporated into REACTPOOL [R.F Kapias, C, Griffiths, J. Hat. Mater.]. Model results indicate that the pool behaviour is strongly affected by the amount of water available for reaction. Surface roughness and wind speed, also have a strong effect on the results, Although there are no experimental data for model validation, it is shown that REACTPOOL gives useful insights into the behaviour of such spills. The paper concludes with a discussion comparing the behaviour for several water reactive chemicals to which REACTPOOL has been applied.