Two melt series, i.e. the MgO–CaO–Al2O3–SiO2 (MCAS) and the Fe2O3–MgO–CaO–Al2O3–SiO2 (FMCAS) series, were investigated with respect to rheological and thermodynamic properties. The results show that substituting MgO for CaO, and Fe2O3 for MgO has a strong influence on viscosity, fragility, configurational entropy and glass transition temperature (Tg) of the glasses. For the MCAS melt series, the effect of the ideal (Ca, Mg) mixing (so-called mixed earth alkaline effect) is seen, and its origin is analyzed. When Fe2O3 is introduced into the MCAS system, both the viscosity and the Tg decrease. The network is weakened by substituting Fe2O3 for MgO. The fragility of the melt decreases with substitution of Fe2O3 for MgO, until it remains unchanged during further substitution. Hence, there is a critical Fe2O3/MgO ratio, above which the fragility is not sensitive to the total amount of Fe2O3 in the FMCAS system.