4-Hydroxy-TEMPO or TEMPOL, formally 4-hydroxy-2,2,6,6-tetramethylpiperidin-1-oxyl, is a heterocyclic compound. Like the related TEMPO, it is used as a catalyst and chemical oxidant by virtue of being a stable radical. Its major appeal over TEMPO is that is less expensive, being produced from triacetone amine, which is itself made via the condensation of acetone and ammonia. This makes it economically viable on an industrial scale. In biochemical research, it has been investigated as an agent for limiting reactive oxygen species. It catalyzes the disproportionation of superoxide, facilitates hydrogen peroxide metabolism, and inhibits Fenton chemistry.