The dissolution and hydration kinetics of MgO single crystals and powder samples were investigated with regard to the H+ and Mg2+ concentrations and the temperature. The rate of dissolution of rotating MgO discs in buffered solutions was determined from measurements of [Mg2+] and those of the crystals and powder fractions were determined by pH and conductivity analysis. The degree of hydration was analysed by means of a thermogravimetric method. Several rate-controlling processes depending on pH were present at room temperature. (1) At pH < 5 the rate-controlling step was proton attack followed by desorption of Mg2+ of OH- depending on the value of [Mg2+]. The rate was proportional to either -pH or pMg-pH. These processes are part of the overall neutralization reaction. MgO + 2H+→Mg2+ + H2O. (2) At pH ≈ 5 the rate-controlling step was a diffusion-limitation process due to protons. The rate was proportional to the proton concentration. (3) At pH > 7 the rate-controlling step was OH- adsorption followed by Mg2+ and OH- desorption leading to a rate maximum. These processes are part of the overall dissolution reaction. MgO + H2O→Mg2+ + 2OH- The neutralization processes are interpreted in terms of irreversible thermodynamics yielding a linear dependence of the rate on pH or pMg-pH. It is concluded from conductivity and scanning electron microscopy measurements during and after hydration experiments that the hydration rate is controlled by the dissolution rate under given conditions. After a supersaturation period Mg(OH)2 precipitates preferentially at the MgO surface, so that an MgO lattice reaction can be excluded. All processes undergo an Arrhenius acceleration with increasing temperature (activation energy, 70 kJ mol-1) and the overall reactions are then limited by proton and OH- diffusion.