Evaporation of potassium from slag was investigated in order to estimate the effect of chemical composition and temperature. The evaporation rate increased with addition of basic oxide as FeO, CaO and MgO and it decreased with addition of amphoteric oxide such as alumina and titanium dioxide. Addition of MgO substituted by CaO under constant basicity B=(CaO+MgO)/SiO₂ affected positively the delayed evaporation of potassium oxide.