The rate of crystallization of (GeSe2)70(Sb2Te3)20(GeTe)10 vitreous samples was investigated by means of differential scanning calorimetry. Crystallization proceeds in two stages. The activation energy of each stage of crystallization was determined. The evolution of the microstructure during crystallization was investigated by X-ray analysis and optical and scanning electron microscopy. Crystalline nuclei emerge in the bulk and develop in a spherulitic form. A lamellar structure is clearly observed when crystallization is completed. © 1983 Chapman and Hall Ltd.