On Sunday 9 June 1996 a violent runaway reaction occurred in a 2.3 m(3) Pfaudler reactor creating a high pressure that led to rupture of the vessel: consequential damage to equipment and buildings was very significant. This paper provides explanations of how and why this accident occurred. At the time of the incident a diazonium ion was being produced for subsequent decomposition to form a phenol. The process had been carried out many hundreds of times before without incident. A combination of isothermal and adiabatic calorimetry was subsequently used to study the thermochemistry of both the desired synthesis reaction, and the undesired decomposition reaction. Plant simulation studies based on this experimental data were then used as an aid to analysing and understanding the events leading up to the accident. In due course the plant was rebuilt and production resumed: some of the strategies used to reduce both the hazards and risks associated with renewed production are discussed. A summary of the overall business impact of the accident is presented.