The objective of this paper is to present the results of the analysis performed on a selection of accidents included in the MARS database. This is a database of past accidents that occurred in Europe maintained by the European Commission, in order to help the Member States to meet the requirements of the Seveso and Seveso II directives. The study is focused on those accidents that involve reactions between chemical substances, whether wanted or unwanted, that generated a hazardous situation by loss of control of such reactions. Runaway reactions are known to be especially dangerous, given that many times they are unexpected, or their possible consequences underestimated, so sometimes chemical industries are not ready to cope with the effects of loss of control of reactive processes. The aim of the analysis is to obtain lessons learned from past accidents in order to prevent similar situations in the future, or to reduce their consequences. Understanding the causes of past accidents, including equipment failures, deviations in the expected course of a reaction, or deficiencies in process operations performed, can help to a better understanding of similar processes. Industries working with potentially dangerous chemicals should consider introducing lessons learned into their safety management systems.