Summary: Comparison of experimental data from the systems MgO-CO2-H2O (closed) and MgO-CO2-A (simulating an open system) shows that the effects of H2O and A on the dissociation of magnesite are almost identical; both behave as inert components reducing the partial pressure of CO2. The dissociation temperature at constant total pressure is lowered according to the proportion of inert volatiles in the initial vapour phase. The dissociation is completed at one temperature (univariant) in an open system but in a closed system it proceeds through a temperature interval (divariant) because the vapour phase changes composition. The amount of dissociation remains small until the upper limit of the interval is reached. More complex dissociation reactions in the systems CaO-MgO-CO2-H2O and CaO-SiO2-CO2-H2O are described; they follow similar patterns. Under closed or partially open metamorphic conditions non-reacting pore fluid components (inert) have to be treated as one additional component for application of the mineralogical phase rule. Comparison of the pattern of metamorphic parageneses with the patterns of reactions occurring under known experimental conditions may provide information about metamorphic processes. Metamorphic reactions can be represented within a petrogenetic model with axes P, T, and pore fluid composition varying between H2O and CO2.