Diesel used to be cheaper than gasoline. Not anymore. Both fuels come from the same crude oil, so why the big price difference? Economics plays a role, but chemistry deserves much of the blame. Especially the surprisingly difficult and expensive business of getting rid of stinky sulfur.

Anyone passing by a gas station lately may have noticed that diesel is considerably more expensive than gasoline. It wasn't always that way. This seems a little strange. Both fuels come from the same barrel of crude oil. So what happened? Did diesel suddenly become more valuable? Did oil companies discover another way to squeeze money out of motorists?

Nope. Economics may call the shots when it comes to fuel prices, but chemistry lurks in the background. And one element is particularly expensive to deal with: stinky sulfur.

Same crude oil, different chemistry

Crude oil is a complex mixture of thousands of chemicals, mostly hydrocarbons (composed of carbon and hydrogen). Gasoline and diesel are both derived from crude oil, but their chemical compositions and (especially) boiling points are quite different.