Co-products are different products that are unavoidably produced simultaneously with each other. This is similar to the concept of by-products. However, a by-product is used when one of the products is considered of lower value and less desirable. Co-products indicates that the products are of similar value and desirability.

Refining is very much a co-product process, as it results in the co-production of a number of high value products simultaneously, such as gasoline, jet fuel and diesel fuel.