Diphosphines, Ph2P(CH2)(n)PPh2 1 (n =1, 2, 3, 4, and 6), were photolyzed by a xenon lamp in air. The P-31 NMR spectroscopic analysis of the reaction showed that 1 is oxidized, according to first-order kinetics, to the monoxide, which is further oxidized to the dioxide. The dependence of the rate constants for the first oxidation on the chain-length n in 1 is interpreted in terms of the orientation of the p-orbitals on the two phosphorus atoms in the intermediate, the diphosphine radical cation.