The X-irradiation of triphenylphosphineborane single crystals yields the (C₆H₅)₃P-BH₂ radical. The ¹H, ¹¹B and ³¹P hyperfine coupling tensors are obtained at room temperature and these show a rapid rotation of the BH₂ group about the P-B axis. These tensors are consistent with a planar structure of the radical. In the discussion the E.S.R. parameters are compared with those of -BH₃, ĊH₃ and (C₆H₅)₃P-ĊH₂.