Abstract The kinetics of the dissolution of a (Na. K) sanidine in CO 2-charged water, were studied experimentally at 200°C as a function of the surface area s and of the time t. The molalities of Na + and K + have been plotted against the product st. As for albite and adularia ( Lagache, Bull. Soc. Fr. Minéral. Cristallogr. 88, 223–253, 1965), the rates of dissolution of Na 2 and K + are smooth continuous functions of st, which implies that they are controlled by the composition of the solution. The comparison between the dissolution of pure sodic or potassic feldspars and that of an intermediate feldspar shows that the sanidine dissolves as if it were composed of albite and K-feldspar grains in the proportions corresponding to its composition. Theoretical considerations presented by Helgeson ( Geochim. Cosmochim. Acta 35, 421–169, 1971; The Feldspars, pp. 184–217, 1972) and Pačes ( Geochim. Cosmochim. Acta 37, 2641–2663, 1973) are reviewed: both had suggested, arguing from my first experimental results, that the dissolution could be described by a process of diffusional mass transfer through a surface layer of reaction products. The present experiments do not agree with such an interpretation of the mechanism of dissolution.