Thermal decomposition, or thermolysis, is a chemical decomposition caused by heat. The decomposition temperature of a substance is the temperature at which the substance chemically decomposes.
Some oxides, especially of weakly electropositive metals decompose when heated to high enough temperatures. A classical example is the decomposition of mercuric oxide to give oxygen and mercury metal. The reaction was used by Joseph Priestley to prepare samples of gaseous oxygen for the first time.
When water is heated to well over 2000°C, a small percentage of it will decompose into OH, monotomic oxygen, monotomic hydrogen, O2, and H2.