The chemical dissolution of lime particles in molten CaO-Al2O3-SiO2-based slags was observed by using a confocal scanning laser microscope (CSLM) in a temperature range from 1450 to 1600 degrees C. The dissolution behavior was found to be largely dependent on temperature and slag chemistry. A considerable increase in the dissolution rate was observed at elevated temperatures. The in situ observations revealed that the dissolution process in the slags containing MgO was controlled by chemical reactions at the interface. For slags without MgO, the dissolution could be divided into three stages when an interfacial reaction (IR) layer formed at the interface. The dissolution was limited by chemical reactions during the early stage, a mixed control of product layer and boundary layer diffusion during the intermediate stage, and boundary layer diffusion control in the final stage. The IR layer formed during dissolution at the interface between the lime particles, and the liquid slag free of MgO was clearly a kinetic barrier for dissolution. A dissolution factor was defined to evaluate the dissolution mechanisms quantitatively by coupling the resistance and the driving forces of dissolution in different slags. This factor provided direct evidence that the addition of MgO in CaO-Al2O3-SiO2 slags is beneficial for lime dissolution.