The triplet-sensitized photorearrangements of 2-phenylallyl phosphites 1, 3, and 4 to the corresponding 2-phenylallylphosphonates la, 3a, and 4a are shown to proceed with complete regioselectivity (5 --> 6 and Scheme 1). A mechanism is proposed in which the 1,2-biradical-like styryl triplet, 9, adds oxidatively to three-coordinate phosphorus to generate a cyclic, triplet, phosphoranyl 1,3-biradical, 10, that undergoes rapid subsequent beta scission to generate product 2-phenylallylphosphonate, 11. Phosphonate la is formed near-quantitatively from dimethyl 2-phenylallyl phosphite, 1. Phosphites 2-4, with phosphorus contained in a five-, six-, or seven-membered ring, respectively, photorearrange much more slowly, although yields of phosphonates 3a and 4a of 50-70% are generated.