Sulfur is an element commonly found in crude oil and petroleum products. Sulfur is considered an undesirable contaminant because, when burned, it generates sulfur oxides. Consequently, most finished petroleum products have a limit on how much sulfur they can contain, making sulfur removal an important part of the overall refinery process. Sulfur can also harm some of the catalysts used in refining process units so it must be removed from some intermediate streams before they can be fed to a conversion unit.

As a general rule, a crude oil grade with high sulfur content will have a lower value. This type of crude is often referred to as a sour crude .

Sulfur can also be mined from underground, naturally-occurring deposits, but this is more costly than sourcing from oil and gas and has largely been discontinued.