Fluid phases play an important role during many processes in the upper mantle and in the crust. They work as efficacious transport agents during mantle metasomatism and depress the solidus of silicates towards lower temperatures. Consequently, they have a prominent influence on the velocities of seismic wave propagation and on the rheological properties of rocks. However, even in modem literature the term 'fluid' is not adequately well defined and it is even used as a synonym for all non-solid phases. One reason for this loose definition is certainly the fact that fluids (especially water dominated ones) and silicate melts show converging similarites with increasing pressure. In many silicate-water systems, the water content of the melt and the silicate content of the fluid increase steadily with increasing pressures. In the silica-water system this behaviour is continued up to a second critical endpoint, where a fluid and a melt are no longer distinguishable and the term 'solidus' has to be reconsidered (Kennedy et al., 1962). Investigations in other simple systems (e.g. diopside-water, Eggler and Rosenhauer, 1978) revealed a similar behaviour, but for more complex mafic and ultramafic compositions a second critical endpoint has due to experimental limitations not yet been determined. The present experimental study started with an inspection of the system albite-water in order to evaluate the accuracy of a new technique to determine major element solubilities in fluid phases and water solubilities in melts. The system albitewater has previously been examined and a consensus with respect to phase relations, solidi and water solubilities in the melts has been achieved. Above 15 kbar the system approaches a second critical endpoint, where a distinction between fluid and melt can no longer be made (Boettcher and Wyllie, 1969; Paillat et al., 1992) and therefore a watersaturated solidus does no longer exist. Therefore it seems to be a suitable system to test our technique, which previously has only been applied to fluid/ mineral trace element partitioning (Stalder et al., 1997). In the next step this technique is being applied to the system MgO-SiO:-H20 at pressures of 60 to 100 kbar and 900 to 1100~ where the transition