A study is reported of the crystallization behaviour of some glass compositions which belong in the system MgO-Al2O3-SiO2 and which lead to vitreous ceramics with α-cordierite as a main phase. DTA, X-ray diffraction and dilatometric analysis were used to investigate the influence of the type and concentration of the nucleation agents. The activation energies of crystallization were calculated and the crystalline phases were identified.