Applying a set of equations derived byGresens (1967), composition-volume relationship are discussed for pairs of soda metasomatites and unaltered enclosing country rocks. A probable volume factor is inferred and the behaviour of elements interpreted. The metasomatic alteration was basically a process of leaching in which two stages can be recognized differing from each other in the prevailing volume factors and in the behaviour of elements. A first stage, during which country rocks were altered into pseudomorphous quartzplagioclase rocks, was characterized by preservation of the volume of the parent rocks; Si and Al were immobile, and the leaching of K, Fe, Mg, Ti and most of the trace elements was balanced by introduction of Na and Ca, accompanied by Sr, Th, Nb and Ta. A second stage, during which pseudomorphous quartz-plagioclase rocks were recrystallized under simultaneous expulsion of quartz and Fe-Ti-oxides, was characterized by a volume decrease of some 30%; Na, Ca, Al and Sr were immobile and most of the trace elements were leached.