The rate of CaO dissolution into slag melts is of great importance in the field of pyrometallurgy. In this study the mechanism of CaO dissolution has been investigated by observing the melt heated on Pt-filament directly through microscope together with a help of X-ray microanalyser to find out a distribution of each element around CaO crystal. The direct observation reveals that the CaO dissolution tends to proceed by the action of molten slag entering between CaO crystal and newly formed 2CaOSiO₂layer through cracks. The effect of slag composition on the formation of 2CaO·SiO₂layer will be shown as well as the effect of halogenides addition on the destruction of this compound layer.