Hydrogenation, hy druh juh NAY shuhn, is a chemical process that adds hydrogen to a substance. Liquid oils are often hydrogenated to produce solid fats. For example, peanut oil is hydrogenated to change it to a solid and improve its odor and flavor. The hydrogenation process is important in refining gasolines. Hydrogen is added to the smaller molecules of gasoline that result when larger gasoline molecules have been cracked by heat and pressure. Hydrogenation is used in making crude oil from coal. In this process, powdered coal is mixed with a little oil and hydrogenated under heat and pressure.