The liquidus temperature, crystallization behavior, and viscosity of calcium–aluminate (CA)-based mold flux systems are investigated by differential thermal analysis, quench tests, and viscosity measurements to understand the impact of SrO, B ₂O ₃, F, and Li ₂O on the flux properties. For mixtures with a CaO/Al ₂O ₃ (wt%) ratio of 1, the liquidus temperature of 1443 K and a viscosity of 0.37 Pa s are achieved with the addition of 6.5 wt% B ₂O ₃. Substitution of CaO for 8.3 wt% SrO in the B ₂O ₃-free compositions increases a liquidus temperature from 1547 to 1559 K, and 2.6 wt% increases the viscosity from 0.35 to 0.41 Pa s, indicating that low amount SrO is not beneficial in adjusting slag properties of CA mold flux. A fluorine content of up to 6.1 wt% decreases the melting temperature from 1547 to 1509 K and inhibits crystallization while promoting volatilization in the liquid state. Li ₂O (2.3 wt%) effectively enhances the glass-forming ability of the CA mold flux and decreases both the liquidus temperature from 1571 to 1509 K and the viscosity of the system from 0.43 to 0.28 Pa s. This study deepens the knowledge on CA flux systems, which could be useful for their effective design.