A large difference of about 5‰ in the O-isotopic composition of quartz characterizes the transition from very low grade or even anchimetamorphic slates to pelites metamorphosed at greenschist facies conditions. A further increase in temperature does not result in systematic isotopic shifts (mean around +15.5‰). However, immediately at the isograds marking the transition from greenschist to amphibolite facies and amphibolite facies to higher amphibolite facies the quartz δ 18O values are significantly higher by >1‰. This increase is restricted to the area immediately at the isograds. It can be shown that during the low grade transformation a fluid phase is present in excess; the metamorphism of this stage therefore can be described as “water present metamorphism”. This fluid phase leaves the system during pervasive deformation of the rocks, during which large scale homogenization processes are possible. At higher temperatures a free fluid phase is only present at specific isograds, where it is produced by mineral dehydration reactions. If the production rate of the fluid phase is sufficiently slow, this results in a re-equilibration of the mineral phases with the newly generated fluid phase, the isotopic composition of which is completely controlled by the primary mineral phases. In intermediate regions with enlarged fluid production and escape rates, a free fluid phase is present only for short periods of time at fractures which form due to fluid overpressure. The high escape rate inhibits the solid phases from re-equilibration with the generated fluid. The term "water absent metamorphism" can be used to describe this state of reduced H2O activity within a rock.