Above 500 K in silicon, isolated hydrogen defect centres scatter intrinsic charge carriers strongly by momentary formation of the neutral atomic ground state, located at the tetrahedral cage centre. This is inferred from the analogous behaviour of muonium, via distinctive muon spin rotation and relaxation signals. A consistent interpretation of a surprisingly large shift of the muon Larmor frequency and the strong transverse and longitudinal spin relaxation rates is achieved in terms of charge-state transition rates into and out of the neutral paramagnetic state. The nature of the charge cycle and the interplay with the crystallographic site are discussed and the electrically active level in the energy gap is determined. (C) 2003 Elsevier B.V. All rights reserved.