The influence of trace impurities is frequently mentioned as a possible or probable cause of accidents in the chemical industry. In process conditions where there is a potential for a fast exothermic decomposition or polymerisation reaction, the contamination of pure chemicals by trace impurities may cause problems. Typical examples of this situation are described concerning the processing of organic nitrocompounds and the storage of reactive monomers, i.e. vinyl acetate and ethylene oxide. In some process instances, i.e. hydrogenation of organic nitrocompounds, trace impurities in the organic substrate are said to have killed the catalyst and caused severe accidents. Many contaminants are known to be catalyst killers in hydrogenation processes. Examples of accidents or runaway reaction hazards caused by trace impurities are given, based on a review of the literature and on our own data. Trace impurities present in common raw materials make their processing more complex and may cause accidents. Two examples are considered, the presence of nitrogen trichloride in chlorine manufacture and the presence of hydrogen sulphide and mercaptans in petrochemical feed-stock. A full literature review is given.