Purity >95%. CYPMPO is a free radical spin trap with excellent trapping capabilities toward hydroxyl and superoxide radicals in biological and chemical systems. Decay of the superoxide adduct of CYPMPO proceeds in an apparent first order fashion with half-lives of 15 and 51 minutes in a UV-illuminated hydrogen peroxide solution and a hypoxanthine/xanthine oxidase system, respectively. CYPMPO traps superoxide radicals generated by bovine neutrophils as effectively as DEPMPO. The high melting point (126°C), low hygroscopic properties, and long shelf-life in aqueous solutions offer significant practical advantages for use of CYPMPO over DEPMPO and DMPO.