This study focuses on the viscosities of synthetic slags used in secondary refining operations. Typically, these slags contain mainly CaO, AI2O3, MgO and Si02. The data for LF slag viscosities are used in a model for a gas-stirred ladle to evaluate the effect of viscosity variations on predicted parameters close to the slag/steel interface. The results show that viscosities of LF slags significantly affect ladle refining operations. It is therefore necessary to control the composition of the synthetic slags carefully during production.