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In order to understand sulfur solubility in the MnO–SiO2–Al2O3–Ce2O3 slag or inclusion system (mol MnO/mol SiO2 = M/S = 2.2 or 0.9, Al2O3 = 14.2[± 1.8] mol%, Ce2O3 = 0–5.6 mol%), the effect of Ce3+ ions on the structure of the Mn–aluminosilicate system with different M/S molar ratios has been studied by micro-Raman spectroscopy, solid-state nuclear magnetic resonance (NMR) spectroscopy and X-ray photoelectron spectroscopy (XPS). Because constant oxygen partial pressure was maintained at p(O2) = 2.8 × 10−7 atm, oxidation state of Mn and Ce would be in Mn2+ and Ce3+ in the MnO–SiO2–Al2O3–Ce2O3 system, which were also confirmed by XPS analysis. Although it was difficult to measure the 27Al solid-state NMR spectra of the quenched MnO–SiO2–Al2O3 system due to the paramagnetic effect, 27Al NMR spectra did show structural changes in aluminate when cerium was added to the Mn–aluminosilicate melts. Addition of Ce2O3 in the high M/S(= 2.2) system caused both an explosive enhancement in the intensity of Raman bands at 600 cm−1 and a peak shift in 27Al NMR spectra, indicating the transition from the [AlO4]–:0.5Mn2+ tetrahedron to a [AlO6]3–:Ce3+ octahedron complex due to a strong attraction between aluminate and the cerium ion. XPS spectra show that the transition of the aluminate structure upon introduction of Ce3+ ions consumes free oxygen in Mn–aluminosilicate melts in high M/S(= 2.2) system. However, the same structural changes were not observed when Ce2O3 was added to lower M/S(= 0.9) system, because Ce3+ ions primarily interact with the pre-existing [AlO6]-octahedron in low M/S system.