The details of the microstructural characteristics of the CaO-SiO2-MgO-Al2O3 (-CaF2) systems solidified during slow cooling from 1600 degrees C, which are critical to understanding the fundamentals of inclusion formation in the stainless steelmaking processes, were investigated using scanning electron microscope and energy-dispersive X-ray spectrometer. Especially, the effect of fluorspar on the solidification behavior of the melts was studied. In addition, the phase equilibria at a specific temperature and composition during cooling cycles were computed using commercial thermodynamic software based on the Gibbs energy minimization principles and were compared with the measured results. In the slag of B = 0.6 containing 30 wt% Al2O3, the relative fractions of spinel and anorthite continuously increase with decreasing temperature from about 1300 degrees to 1150 degrees C, followed by constant values at temperatures less than about 1100 degrees C. In the liquid slag of B = 1.0, the spinel phase appears at temperatures greater than about 1350 degrees C, followed by the formation of melilite and their relative fractions increase with decreasing temperatures from about 1300 degrees to 1200 degrees C. The anorthite appears at temperatures less than about 1200 degrees C. The calculated results could explain the observed phenomena. In the system of B = 0.6 containing 10 wt% CaF2 and 30 wt% Al2O3, the spinel phase was calculated to be in equilibrium with anorthite and merwinite. However, anorthite was not observed in the experiments. This could be understood from the changes in melt composition because of formation of gaseous fluoride SiF4. The observed phenomena and the calculated results commonly indicated that the amount and the size of spinel crystallized in the slag (B = 1.0, 5 wt% CaF2) increased on increasing the content of alumina from 10 to 30 wt%. The size of spinel in the slag of B = 1.0 containing 30 wt% Al2O3 decreased, and thus the number of spinel per unit volume increased with increasing content of fluorspar up to 10 wt%. The shape of spinel changed from polygonal to irregular (or broken) type on increasing the content of fluorspar.