- A chemical reaction that takes place between an oxidizing substance and a reducing substance. The oxidizing substance loses electrons in the reaction, and the reducing substance gains electrons. For example, rust forms when there is a redox reaction between oxygen contained in water or moist air (an oxidizing substance) and iron (a reducing substance). Redox reactions also occur when glucose (a type of sugar) and fat are broken down in the body to make energy. Also called oxidation-reduction.