UV Photolysis (lambda = ca. 254 nm) of Ni(CO)(4) molecules isolated in O-2-doped argon matrices at ca. 20 K has been monitored by reference to the IR spectra of the matrices. These spectra indicate that Ni(CO)(4) undergoes photooxidation to give four distinct nickel carbonyl intermediates A-D with the release of CO and CO2, ultimately to arrive at the final product (eta(2)-O-2)(2)Ni. The stoichiometry and likely structure of each intermediate A-D have been deduced by analysis of the number, energies, and relative intensities of its IR absorptions, with particular reference to the response to (CO)-C-13- and O-18-enrichment. Hence, the following identities have been inferred : A, Ni(CO)(3); B, (eta(2)-O2)Ni(CO)(2); C, (O-2)Ni(CO)(3); and D, O=Ni(CO)(2). The system exhibits some photoselectivity, and reversal of the reactions leading to A-D can be effected under the action of UV radiation with wavelengths near 313 nm or by annealing the deposit to ca. 30-35 K. A partial mechanism is proposed to account for the course of the matrix reactions.