Carbon rejection refers to any chemical process that results in the production of elemental carbon (pet coke) and therefore a reduction in carbon content (rejection) in the liquid hydrocarbons involved.

Carbon rejection is common in most cracking processes in refining such as the FCC, RCC and coker where excess hydrogen is not available to bond with the carbon atoms separated in the cracking process. Carbon rejection does not occur in hydrocracking processes, which gives them an advantage in overall liquid product yield.