Dissolution of fayalite has been monitored in a small size batch reactor at a constant pressure of 30 MPa and temperatures of 50, 65, 80 and 100 °C, either in dilute HCl or in acetate buffered solutions. The technique, based on in situ X-ray absorption spectroscopy measurements, allowed simultaneous measurements of aqueous iron concentration and speciation as a function of time under high pressure and high temperature conditions. Far from equilibrium, dissolution rate normalised to geometric surface area (Rnet) can be described in dilute HCl solutions by: R net = k 0 exp − E a / R T a H + n with log(k0) = 0.17 ± 0.63 (k0 being expressed in mol cm− 2 s− 1); Ea = 45,100 ± 4200 J mol− 1; n = 0.74 ± 0.15, so that Rnet is expressed in mol cm− 2 s− 1. These results are found to be consistent with some previous fayalite dissolution studies. In acetate buffered solutions, the dissolution rate was found to be enhanced and positively correlated with the activity of acetate. Whereas speciation of Fe2+ in HCl solutions is dominated by hexahydrated octahedral species, the combination of ab initio XANES calculations, measurement of X-ray absorption spectra, and thermodynamic modelling allowed us to determine that aqueous Fe2+ was complexed octahedrally with two acetate ligands, each bonded to iron through one oxygen atom, the corresponding formula being Fe(CH3COO)2*4H2O. Supposing that surface complexes resemble the major complex species in solution, this solvation by acetate ions would explain the observed enhancement of the dissolution rate, because of the nucleophilic attack of acetate ligands at the silicate surface.