Alkanes are saturated hydrocarbons with between 5 and 40 carbon atoms per molecule which contain only hydrogen and carbon. The light distillates range in molecular composition from pentane (5 carbons: C5H12) to octane (8 carbons: C8H18). Middle distillates range from nonane (9 carbons: C9H20) to hexadecane (16 carbons: C16H34) while anything heavier is termed a heavy distillate. Hydrocarbons that are lighter than pentane are considered natural gas or natural gas liquids (liquefied petroleum gas).

A few further refinement processes are described below:

•  Desalting removes salt from crude oil before entering fractional distillation.

•  Desulfurization removes sulfur from compounds, and several methods are possible. Hydrodesulfurization is the typical method, and uses hydrogen to extract the sulfur. This occurs after distillation.