Methods are given for the design of dump tanks to separate the liquid and vapour/gas phases from a two-phase emergency relief discharge from a runaway chemical reactor. A small-scale test apparatus is described which has been used to evaluate the design methods. Results and analysis are given for reacting and non-reacting systems, which give good agreement with the predictions from the design methods for water depressurization and a ＇vapour-pressure＇ system. Results for a hybrid system indicate the problems associated with deciding whether the major component of the relief stream is the gas or the vapour.