At very low fluid/rock mass ratios the hydrothermal alteration process corresponds to isochemical recrystallisation of the primary rock. The resulting full equilibrium assemblage with the composition of an average crustal rock contains the phases albite, K-feldspar, K-mica, biotite, quartz and (depending on temperature) epidote, prehnite or one of the Ca-zeolites. Relative Na +, K +, Mg 2+ and Ca 2+—solution activities in such a rock-dominated alteration system are uniquely fixed and provide useful reference points with regard to the degree of attainment of full fluid/rock equilibrium. With increasing fluid/rock mass ratios the composition of now increasingly fluid-dominated alteration assemblages is determined by the interplay of three major processes: hydrogen metasomatism as a function of CO 2 reactivity increasing with the horizontal distance from major fluid upflow zones and leading to the formation of Al-enriched alteration assemblages; potassium metasomatism accompanied by silicification in or close to major fluid upflow zones leading to potassic and phyllic alteration; sodium, magnesium, calcium metasomatism associated with descending and heating solutions leading to propylytic alteration of recharge zones. Two new parameters, reactivity and exchangeability, determining the effectiveness of fluid components with respect to hydrothermal alteration are introduced.