The effect of pH and Gibbs energy on the dissolution rate of a synthetic Na-montmorillonite was investigated by means of flow-through experiments at 25 and 80 degrees C at pH of 7 and 9. The dissolution reaction took place stoichiometrically at 80 degrees C, whereas at 25 degrees C preferential release of Mg over Si and Al was observed. The TEM-EDX analyses (transmission electronic microscopy with quantitative chemical analysis) of the dissolved synthetic phase at 25 degrees C showed the presence of newly formed Si-rich phases, which accounts for the Si deficit. At low temperature, depletion of Si concentration was attributed to incongruent clay dissolution with the formation of detached Si tetrahedral sheets (i.e., alteration product) whereas the Al behaviour remains uncertain (e. g., possible incorporation into Al-rich phases). Hence, steady-state rates were based on the release of Mg. Ex situ AFM measurements were used to investigate the variations in reactive surface area. Accordingly, steady-state rates were normalized to the initial edge surface area (11.2 m(2) g(-1)) and used to propose the dissolution rate law for the dissolution reactions as a function of Delta G(r) at 25 degrees C and pH similar to 9: r(mol m(-2) s(-1)) = 3.82 x 10(-12) [1 - exp(-8.23 x 10(-10)vertical bar Delta G(r)/RT vertical bar(5.47))].