We present a study of the hyperfine structure in the electron spin resonance spectra of phosphorus-doped a-Si0·7Ge0·3: H. Electron bombardment and annealing are used to vary the defect density and to shift the Fermi level in the range 0·45<EC–EF<0·75 eV. The main observation is that electron bombardment causes an enhancement of the density of hyperfine centres and a major decrease of the hyperfine splitting from 265 to 200G. The data show that phosphorus introduces two hyperfine centres: the well known fourfold coordinated phosphorus atoms Pi and a deep state the density of which can be enhanced by electron bombardment and hydrogen effusion. We suggest that the latter is a dangling-bond state on a twofold coordinated phosphorus (P02).