The influence of basicity, heat treatment as well as different oxygen partial pressures on the phase relationships in the CaO-MgO-SiO2-Cr2O3 slags was studied with a view to control the precipitation of Cr-spinel in the slag phase. The equilibrium phases in CaO-MgO-SiO2-Cr2O3 slag system in the range on 1673-1873 K have been investigated under low oxygen partial pressure as well as in as air atmosphere. In low oxygen partial pressure experiments, a suitable mixture of CO and CO2 was used to control the oxygen partial pressure. The oxygen partial pressure was kept at 10-4 Pa. The Cr2O3 and MgO contents in the slag were fixed to be 6 and 8wt% respectively. The basicity (CaO/ SiO2) of the slag was varied in the range 1.0-2.0. Gas/slag equilibrium technique was adopted to synthesize the slag at a suitable temperature above the liquidus point. One heat treatment procedure is that the samples were heated to and soaked at 1873 K for 24h in order to achieve the equilibrium state and subsequently quenched in water. The other is that the samples were heated to and soaked at 1873 K for 24h, then slow cooled to 1673 K and soaked at this temperature for additional 24h in order to achieve the equilibrium state at lower temperature before quenching in water. The chromium distribution and phase compositions in the quenched slag were studied using scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS) and X-ray diffraction techniques (XRD). FACTsage software was used for the phase equilibrium calculations. The experimental results obtained from the present work are compared with the calculation results from FACTsage software as well as with results from samples directly quenched after soaking at 1873K. It is found that the spinel formation at 1873 K in air atmosphere is favored in the slag basicity range of 1.0 to 1.6. The size of spinel crystals increased drastically after slow cooling followed by annealing compared to samples being quenched after soaking at 1873 K. The amount of foreign elements dissolved in the spinel phase, and matrix phases decreased after slow cooling followed by annealing at lower temperature, resulting in purer phases with less defects. It was found that the amount of foreign elements in the spinel phase, and other phases decreased after soaking at very low PO2. The size of the spinel crystals was found to be larger in samples with low basicity. Spinel phase precipitation has improved in the samples with higher basicities compared to the results obtained in air. / QC 20111208