This paper discusses four different chemical processing accidents that resulted from plant mal-operations. Each incident was relatively minor yet provided valuable information about the stability and/or vulnerability of the process to upset conditions. The reaction classes involved were: Polyester polymerization Secondary amine formation followed by decomposition Oxidation of poly-alkylated aromatic hydrocarbon Base catalyzed oligomerization The paper describes the ＇after-the-fact＇ data that was obtained by calorimetric hazards testing. We will discuss how the information was used, in each case to improve the process safety of the particular operation. We also describe an incident database that may be used by chemical companies to provide an on-going ranked and categorized record of accidents and incidents that have occurred. The value of the database information lies in the fact that the actual and potential severity of each accident is estimated from the data of the incident events. This information can be used to distinguish processes that may be responsible for the majority of the minor incidents from those processes where more severe, but infrequent, accidents occur. The information is also valuable in providing a focus with which to drive process safety improvement efforts.