Abstract: Pollucite is a silicate mineral of the rare element caesium, occurring in granitic pegmatites. Experiments have been carried out at 450, 600, and 750°C, 1.5 kbar, to study the equilibrium between pollucite, albite and the co-existing hydrothermal solution. When pollucite co-exists with albite, the alkaline composition of the solution is buffered. The Cs/Na ratio of the solution has been determined to be 0.11 at 450°C 0.22 at 600°C and 0.23 at 750°C. Pollucite contains about 15 mol.% of sodium, whereas albite is almost purely sodic. In nature, pollucite with more than 82 mol.% caesium has never been found. This can be explained by the absence of solutions in granitic pegmatites having a higher Cs/Na ratio than those determined by us.