Stilbite samples from two localities were examined. Preliminary D.T.A. and T.G.A. investigations to 1 400 °C were followed by high temperature diffractometry. Debye–Scherrer and Guinier techniques were used to study cooled samples, Diffractometer and single crystal data were processed with a computer to calculate cell parameters and thermal expansion.Stilbite changes to a metastable form of epistilbite at 145 °C. At 240 °C and 470 °C epistilbite shows two modifications followed by gradual destruction. Semi-amorphous silica formed by this process yields β-cristobalite and anorthite at 920 °C. Anorthite is replaced, at 1 340 °C, by an unidentified cubic phase. β-Cristobalite is found to be metastable down to room temperature; its room temperature cell parameter "a" is inversely proportional to the maximum temperature of annealment.