Crystallisation and annealing behaviour of two caesium borosilicate glass samples with approximate boroleucite composition were characterised by means of differential scanning calorimetry measurements as well as X-ray powder diffraction investigations with subsequent profile fitting of the observed patterns according to the Rietveld and/or Pawley method. While one sample crystallised primarily in an orthorhombic phase ( = 6.592(3) Å, = 11.825(5) Å, = 12.620(6) Å; possible space groups: ((26), setting: ), 2 (28) and ((51), setting: )) which has been unknown until now and transformed irreversibly into the well-known cubic boroleucite phase at ≈ 850 °C, cubic caesium boroleucite (space group: 3̅ (230)) was crystallised directly from the other sample. In this case, the orthorhombic phase was metastable. At the onset temperature of glass crystallisation = 770 °C, it existed only for about 75 min. The change of the lattice parameter of the cubic unit cell of caesium boroleucite in the course of the thermal treatments was explained by the alteration of the size of the TO tetrahedra (T = Si, B).