The control of major accident hazards linked with the storage and processing of dangerous substances in chemical and petrochemical installations is regulated in the European Union by the so-called "Seveso II Directive." One of the requirements in this Directive is the declaration of not only the hazardous substances as present onsite, but also any hazardous products that could form during a loss of control situation, This study focused on the development of an experimental technique to determine the substances that might be formed during an uncontrolled chemical reaction or runaway reaction. The decomposition reaction of a diazo compound was studied with the technique to assess its applicability. The results show that, apart from its applicability in relation to the Seveso H Directive, the same technique can be used to obtain data for the design of gas treatment systems or to study the mechanism behind runway reactions. Understanding this mechanism, in turn, helps to identify conditions that might favor the occurrence of, or might temper the course of, the runaway reaction.