in petroleum refining, process by which heavy hydrocarbon molecules are broken up into lighter molecules by means of heat and usually pressure and sometimes catalysts. Cracking is the most important process for the commercial production of gasoline.

Cracking of petroleum yields light oils (corresponding to gasoline), heavier oils, and such gases as methane, ethane, ethylene, propane, and propylene. These gases are important chemical raw materials and are the starting point for the production of a large number of compounds that constitute five major groups of end products: synthetic rubber, plastics, textiles, detergents, and agricultural chemicals.

Thermal cracking processes came into use in 1913 for breaking up large nonvolatile hydrocarbons to obtain gasoline. In 1936 the cracking process was improved with catalysts to obtain a higher-octane product. Gas oils are cracked to produce high-octane gasoline in the presence of a clay catalyst. See also alkylation.