The effect of the CaO/SiO₂ molar ratio on the surface tension of calcium aluminosilicate melts containing magnesia (CaO–SiO₂–Al₂O₃–MgO) has been explored using a ring method at 1723–1823 K; the Al₂O₃ and MgO contents were approximately 12 and 8 mol%, respectively. The CaO/SiO₂ molar ratio of the samples was varied in the range of 1.1–1.7. The surface tension of the CaO–SiO₂–Al₂O₃–MgO system simultaneously increased upon increasing the CaO/SiO₂ molar ratio. The present data were compared with the surface tension of the binary calcium silicate (CaO–SiO₂) and the ternary calcium aluminosilicate (CaO–SiO₂–Al₂O₃) melts reported in the literatures. The surface tension of the present CaO–SiO₂–Al₂O₃–MgO melts was higher than those of the binary calcium silicate melts and slightly lower than those of the ternary calcium aluminosilicate melts when the polymerization degrees of the melts were comparable. The change in the surface tension was considered from the viewpoint of the local structure of oxygen atoms at the melt surface. Oxygen atoms, which require higher coordination by cations in the bulk, may tend to lose their neighbors at the surface of the melts, which can result in the formation of unsatisfied bonds at the surface. An increase in the number of unsatisfied bonds can yield an increase in the surface tension.