Thermolysis is chemical decomposition by heating. If a reaction is endothermic, heat is required for the reaction to proceed. If the reaction is exothermic, the reaction proceeds spontaneously after initiation, which sometimes causes explosion. The term pyrolysis is often used in place of thermolysis when organic compounds are decomposed at high temperature. Pyrolysis gas chromatography (Py-GC) and pyrolysis GC/MS are used to characterize organic polymers. Thermolysis of petroleum is called cracking.