Rare-earth elements (REEs) are a group of 17 chemically similar metallic elements (i.e. 15 lanthanides, plus scandium and yttrium), which are strategic for application in modern life. Both the United Nations and the European Union classified REEs as critical and strategic metals due to their properties and their increasing use in critical applications such as wind power plants. Due to large and increasing domestic demand, China, the largest producer of rare earths, has tightened export quotas for rare earth concentrates since 2012. Recycling rare earths from secondary resources such as slag is particularly important to increase resource efficiency, escape scarcity and minimize environmental impacts. The common method for processing REEs is hydrometallurgy. However, the disadvantages of this method are the need for finely ground concentrate and high wastewater generation, which causes significant environmental pollution. Therefore, this method is not recommended for direct recovery of REEs from slags with low REE content. The formation of RE-rich phases by pyrometallurgical processes, such as heat treatment, which are separable from REE-free phases and leachable can be used to enrich RE in the concentrate. The objective of this research is to provide the understanding of the rare earth elements (REEs) extraction process from slag, including the ability of various solid phases to incorporate REEs. This study applied cerium oxide, CeO2, with a purity of 99.9 % as a representative of rare earth oxides (REOs) as they have similar properties both physical and chemical. In addition, several other substances, namely CaO, MgO, SiO2, and Al2O3, were also used in some experiments as constituent components of artificial slag. The experiment focused on the dissolution of cerium oxide in the phases of the system CaO-MgO-SiO2(-Al2O3). In this investigation, used contents of CeO2 were 1 wt.-%, 2 wt.-%, and 5 wt.-%. After determining the initial compositions, the slag sample were prepared and then melted at elevated temperature until the transformation from solid phase to the liquid phase was reached and observed using the hot stage microscope (HSM) with various parameters. Temperature used to melt the slag samples were 1400 oC, 1550 oC, and 1600 oC whereas the holding time of the temperature was varied between 15 minutes and 1 hour. Afterwards, an analysis by a SEM instrument of JEOL JSM IT-300 LV, equipped with energy dispersive X-ray (EDS) served for the determination of type and compositions of the generated phases. The result shows cerium can be enhanced by forming a Ce-incorporating phase, for instance Ce9.33-xCax(SiO4)6O2-x containing about 58.82¿62.85 wt.-% cerium oxide. Lower cerium oxide contents can also be found in some phases such as Ca2-xCexSiO4+¿, CaSiO3, CeCa3Si6O17, (Ca,Mg)SiO3, (Ca,Mg,Ce)(Al,Mg)(Al,Si)2O6. In the tests for the system CaO-MgO-SiO2 with an initial cerium oxide content of 5 wt.-%, the highest cerium oxide content was reached with 62.85 wt.-% in the phase Ce9.33-xCax(SiO4)6O2-x at 1600 oC for 15 min with a basicity of 1.38. When the basicity is reduced to 0.52, the generated phases are dominated by SiO2 due to its high content in the initial composition while cerium oxide remains in the matrix. No separation into several phases can be observed from the experiment carried out at basicity of 0.67 as a single homogenous phase resulted. At lower initial CeO2 contents of 2 wt.-% and 1 wt.-%, no Ce-incorporating phase forms for an increased melting time of 1 hour. When Al2O3 is added with a fixed content, cerium oxide can be either incorporated in a certain phase such as (Ca,Mg,Ce)(Al,Mg)(Al,Si)2O6 (20.86 wt.-% Ce2O3) or remained in the matrix (1.72-5.79 wt.-% Ce2O3).