An unexpected highly exothermic reaction followed by explosion has been studied by means of reaction calorimetry and adiabatic calorimetry. The investigation of the process shows that the incident took place by a local adiabatic temperature rise in the reactor vessel due to the presence of potassium carbonate, which activated the decomposition of a nitro compound with an unusual low temperature onset. The study is completed with an improved and safer procedure consisting of changing the order of chemicals addition as an alternative approach.