Annealing at 400 ≤ T ≤ 600 °C (40 ≤ P ≤ 60 MPa and HM buffer-controlled f(O2)) of tourmalines synthesised at the same T and P with NNO buffer induces an oxidation of Fe2+ into Fe3+ in some Y sites only (Mössbauer Spectroscopy data). Annealing in the same conditions of natural tourmalines is consistent with these results. FTIR spectroscopy shows that oxidation of Fe2+ into Fe3+ is charge-balanced by deprotonation of the external OH(3) groups. © 2002 Académie des sciences / Éditions scientifiques et médicales Elsevier SAS.