Water-spray systems have been designed for mitigating accidental outdoor releases of water-soluble gases. This paper describes the new application of water systems to mitigating indoor releases. The performance of spray systems is studied by using models of atmospheric release and spray mitigation, having adjusted them for the indoor environment. A case study is presented of a mitigation system for a railcar-tank HF unloading station, designed to absorb HF released through an open 1 in. valve, with the highest achievable effectiveness. We considered HF releases from the tank at different temperatures and pressures, and spray systems with different type of nozzles at different elevations. The type of release, flow rates and airborne concentrations were determined by the HGSYSTEM package of models and mass-balance calculations. The effectiveness of HF removal of alternative deluge systems was estimated using the model HGSPRAY5. This effectiveness ranged from 64% to 98%, depending on the conditions of the release, the water flow rates, and the type and positioning of the nozzles. Published by Elsevier Science Ltd.