A structurally-based viscosity model (using a few optimized parameters) is proposed to represent viscosity as functions of both temperature and composition for the CaO–MgO–Al₂O₃–SiO₂. The model represents the slag structure through the different types of oxygen ions formed in the melt. Approximate methods for calculating the concentrations of these different types of oxygen ions are proposed and are then used to describe the effect of melt structure on viscosity. The model provides a good description of the viscosity behavior varied with composition and temperature within the CaO–MgO–Al₂O₃–SiO₂ system. This includes pure systems: Al₂O₃ and SiO₂; binary systems: CaO–SiO₂, MgO–SiO₂, Al₂O₃–SiO₂ and CaO–Al₂O₃; ternary systems: CaO–MgO–SiO₂, CaO–Al₂O₃–SiO₂ and MgO–Al₂O₃–SiO₂; quaternary system: CaO–MgO–Al₂O₃–SiO₂. The different roles of CaO and MgO on viscosity are also discussed; these tend to differ in melts with or without Al₂O₃. In the absence of Al₂O₃, CaO reduces the viscosity more than MgO. In contrast, when Al₂O₃ is present, the Ca²⁺ ions take priority over Mg²⁺ ions in the charge-compensation of Al³⁺ ions which leads to the formation of more stable Ca–AlO₄^5– tetrahedral structures, which, in turn, results in an increase in viscosity. However, when there is enough basic oxide (CaO or MgO) present to generate non-bridging oxygens, CaO reduces the viscosity more effectively than MgO.