The catalytic cracking process, which first came into prominence during the recent war, was one of the means adopted by the petroleum industry to meet the demand for aviation spirit of high quality to fuel the Allies' air forces. It was applied mainly in the United States and quantitatively made the largest contribution of any process. The process is a modification of the older thermal cracking process, the presence of the catalyst bringing about at a lower temperature the breakdown of large hydrocarbon molecules into smaller ones (some in the petrol boiling range) whilst at the same time promoting side reactions which improve the quality of the spirit.