Irradiation of diisobutyryl peroxide isolated in an argon matrix with ultraviolet light produces the isopropyl radical and carbon dioxide. Further irradiation of the isopropyl radical with ultraviolet light of a shorter wavelength produces an organic acid, ester, and an olefin. Formation of these products is conveniently explained on the basis of an excited radical reacting with carbon dioxide. In addition to photochemical studies, the infrared spectrum of the isopropyl radical is reported for the first time. Infrared and theoretical studies both support that the isopropyl radical exists in more than one conformation in rare gas matrices. © 1980 American Institute of Physics.