Sour crude oil contains the impurities hydrogen sulfide (H2S) and carbon dioxide, or mercaptans. All crude oil contains some impurities. When the total sulfide level in the oil is 1 % the oil is called "sour."

The impurities will need to be removed before this lower quality crude can be refined into gasoline, thereby increasing the cost of processing. This results in a higher-priced gasoline than one made from sweet crude oil. Sour oil is toxic and corrosive, with high levels of hydrogen sulfide. The oil has the smell of rotten eggs, and at high concentrations the inhalation of hydrogen sulfide is fatal.

The name given to barrels of crude oil that meet certain content requirements, such as low levels of sulfur and hydrogen.

Sweet crude future contracts are the most popular oil contracts traded on commodity markets. This type of oil is much easier to refine than sour crude.

Sweet crude oil is a type of petroleum. The adjective sweet refers to small amounts of hydrogen sulfide and carbon dioxide; sweet crude generally contains less than 0.5% sulfur. This high quality, low sulfur crude oil is commonly used for processing into gasoline and is in high demand, particularly in the United States and China.

Producers of sweet crude oil include Romania, Sudan, United Kingdom (Brent Crude), United States (West Texas Intermediate), Oman, Yemen, Nigeria, Malaysia and Canada.