A series of hydrothermal experiments were performed to determine the phase relations on the beidellite-saponite pseudo-binary join. Quenched glasses with stoichiometric dehydrated compositions of an Na-rich smectite on the join were heated at 250-500 degrees C for durations of 1-151 d at 100 MPa. Time-temperature diagrams showed that immiscibility occurs between dioctahedral smectite (beidellite) and trioctahedral smectite (saponite) below 400 degrees C. Thus, smectite with intermediate chemical composition was considered as metastable in this system. Above 400 degrees C, the assemblage of regularly interstratified saponite-chlorite, quartz, and feldspar was recognized in the intermediate chemical compositional region of this join. On the beidellite side of this join, beidellite and mixed-layer phases of smectite and a regular interstratification of montmorillonite-beidellite, are possible phases that occur at <300 degrees C. They readily reacted to form a mixture of dioctahedral rectorite plus quartz at 300 degrees C. This assemblage then reacted to a dioctahedral "mica", which can expand with glycol and quartz. On the saponite side of this join, a single phase, saponite, existed at <400 degrees C, and transformed to saponite plus trioctahedral rectorite with aging and increasing temperature of synthesis. The alteration was affected strongly by the chemical composition of the binary system.