The kinetics of the interfacial reaction of H2O on liquid silica-saturated iron silicates has been studied by means of measurement of the HDO-H2 isotope exchange reaction. The apparent first order rate constant, ka, was found to be inversely proportional to the H20/H2 ratio, consistent with two electron (melt to adsorbed/dissociating molecule) transfer mechanism and can be given by the expression In ka = -1.97 x 104/T + 1.57 - ln(pH2O/pH2), where the units for ka are mol cm -2 s-1 atm 1. The results of the current study are shown to be consistent with the indirect measurements of earlier work.