Calculations of the thermodynamic activity of aluminum and calcium oxides in the CaO–Al2O3 and CaO–SiO2–Al2O3 systems have been performed using the representations of the polymer model of oxide melts, taking into account the variable functionality of the monomer. When carrying out calculations using the equations of the polymer theory, the polymerization constants were found in binary oxide systems “modifier oxide–complexing agent oxide”. The activity of aluminum oxide was considered as “effective”, which was determined taking into account the fraction of aluminum located in the tetrahedral cavities between the four oxygen ions and, accordingly, forming the AlO5−4 anions, and in the octahedral cavities between the six oxygen ions, which are present in the melt in the form of the Al3+ cation, acting as a modifier. In the calculations of the activity of aluminum and calcium oxides in the CaO–Al2O3 system, it was assumed that aluminum with a molar fraction X of calcium oxide less than 0.7 is present in quadruple coordination and exhibits acidic properties. At high CaO contents, aluminum exhibits both acidic and basic properties. In the CaO–SiO2–Al2O3 slags, agreement between the calculated and experimental values of the activity of the components was achieved by varying the proportion of aluminum in the quaternary and sixfold coordination with respect to oxygen. It was found that the dependence of the activity of aluminum oxide on the molar fraction of silicon dioxide has an extreme character with a maximum at XSiO2 = 0.3. The revealed tendency is explained by the fact that with an increase in the proportion of silicon dioxide to 0.3, the amount of Al3+, which has large degrees of freedom and characterizes the manifestation of basic properties, increases, and the presence of aluminum in fourfold coordination decreases, which affects the manifestation of acidic properties. With an increase in the molar fraction of SiO2 to more than 0.3, aluminum becomes more able to form its own isolated complexes AlO5−4 and, accordingly, the activity of aluminum decreases due to an increase in its bonding with oxygen atoms, and again the acidic properties of Al2O3 prevail.