A significant challenge for operation of entrained-flow coal gasifiers is the crystallization that occurs inside the liquid slag, which results in an increase in viscosity and slower flow of the slag along the wall. The chemical composition of slag is an intrinsic factor in determining crystallization characteristics. In this study, nine synthetic slags with various CaO concentrations (545%) were investigated for their crystallization tendency. Differential scanning calorimetry (DSC) and observations using the single hot thermocouple technique (SHTT) were combined to measure the influence of CaO on the crystallization temperature and crystalline morphology and to construct temperaturetime transformation (TTT) diagrams. The crystalline phases were determined using scanning electron microscopy (SEM) with an energy-dispersive X-ray detector (EDX) and were compared to phases predicted by FactSage simulations. For the prepared slags, the wide range of CaO concentrations considered crossed through several stable crystalline phases. The resulting influence on crystallization behavior and degree of crystallization is consequently not linear or proportional to the CaO content. Slags with 1535% CaO had a higher crystallization tendency and lower crystallization temperature than slags with lower CaO contents. Diopside and anorthite were the two main crystalline phases for slags with less than 35% CaO. However, when the content of CaO exceeded 40%, calcium tended to specifically combine with silica to form Ca2SiO4, which resulted in high crystallization temperatures.