Hydrodealkylation is a chemical reaction that often involves reacting an aromatic hydrocarbon, in the presence of hydrogen gas to form a simpler aromatic hydrocarbon that does not have the functional groups. An example is the conversion of 1,2,4-trimethylbenzene to xylene. This chemical process usually happens at high temperature, at high pressure, or in the presence of a catalyst. The catalyst are mainly transition metals, such as chromium or molybdenum.