A study of the crystallization behaviour of synthetic slags in the Ca0-Mg0-Al2C3-SiO2 system has been carried out using iron sulphide and chromium oxide as nucleating agents. The experiments based on iron sulphide were equilibrated at temperatures of 1350°C and 1400°C under controlled sulphur and oxygen potentials. Crystallization studies using X-ray diffraction and optical microscopy as the principal techniques showed that under mildly reducing conditions, iron sulphide induced nucleation of diopside occurred to give a finely crystalline product. Consideration was given to the compositions. heat treatments and sulphur and oxygen potentials ever which this result could be obtained. Iron sulphide precipitation results were shown to be consistent with a homogeneous nucleation model using the available thermodynamic data for iron sulphide in silicate melts. A number of glasses based on the composition (Wt.%) 53 SiO2 15 A1203 32 Ca0 with up to ten Wt. % CaO replacement by MgO were melted at 1500°C together with varying amounts of up to 0.8 Wt.% Cr203 under argon and oxygen.