Process safety management guidelines suggest that a facility operator should investigate and document a plan for installing systems to detect, contain or mitigate accidental releases if such systems are not already in place. In addition, proposed EPA 112(r) regulations-would require such investigation. This paper illustrates how mathematical modelling can aid such an evaluation. It describes how the HGSPRAY and HGSYSTEM models can be used to study the effectiveness of water-spray mitigation systems. The HGSPRAY model is a two-dimensional spray model that describes absorption of gases by water sprays, air entrainment and heat transfer. The HGSYSTEM models describe several stages of an accidental gaseous release, including depressurization, phase change, and atmospheric dispersion of buoyant or denser-than-air gases. The HGSPRAY model has been independently verified with experimental data involving releases of hydrogen fluoride. The HGSYSTEM models have also been independently verified by comparisons with a wide range of experimental databases. Recent developments in the modelling of mitigation systems include: (i) extension of the HGSPRAY model to NH3; (ii) linkage between boundary layer fluid modelling and mathematical modelling; (iii) an interface between HGSPRAY and HEGADAS; (iv) a two-dimensional representation of monitors spraying in an actual (three-dimensional) configuration; and (v) modelling of a release in an industrial set-up with monitors encircling the point of release. Two hypothetical cases are presented as examples of how these models can aid the design of water spray mitigation systems.