The catalytic cracking process, which first came into prominence during therecent war, was one of the means adopted by the petroleum industry to meetthe demand for aviation spirit of high quality to fuel the Allies' airforces. It was applied mainly in the United States and quantitatively madethe largest contribution of any process. The process is a modification ofthe older thermal cracking process, the presence of the catalyst bringingabout at a lower temperature the breakdown of large hydrocarbon moleculesinto smaller ones (some in the petrol boiling range) whilst at the same timepromoting side reactions which improve the quality of the spirit.