The solubility and the dissolution mechanism of Ru in the CaO–SiO₂, Na₂O–SiO₂, and Na₂O–SiO₂–Al₂O₃ slag systems have been investigated in various conditions in order to understand the Ru dissolution behavior during pyrometallurgical recovery. The Ru solubility increases with an increase in the oxygen partial pressure and content of basic oxides; however it decreases with an increase in temperature. This implies that Ru dissolves in slag as an acidic oxide. The dissolution reaction is found to be Ru+3/4O₂+1/2O²⁻=RuO₂⁻ and ΔH^°=−130±20 kJ/mol. From the dissolution behavior, Ru loss into the slag during Ru recovery by Cu smelting has been estimated to be about 50 ppmw.