The presence of non-metallic inclusions (NMI) such as sulphides and oxides may be detrimental to the control of the steel casting process and product quality. The need for their identification and characterization is, therefore, urgent. This study uses time-gated Raman spectroscopy for the characterization of synthetic duplex oxide-sulphide phases that contain CaS and the oxide phases of Al₂O₃, CA, C12A7, C3A, and MgO·Al₂O₃ (MA). Binary phase samples of CaS–MA, C3A–CaS, C12A7–CaS, Al₂O₃–CaS, and MA–CaS were prepared with varying phase contents. The relative intensities of the Raman peaks were used to estimate the samples’ phase content. For a quantitative estimation, linear regression calibration models were used to evaluate the change in phase content in the samples. The most suitable Raman peak ratios had mean absolute error (MAE) values ranging from 3 to 7 wt. % for the external validation error, and coefficients of determination (R²) values between 0.94 and 0.98. This study demonstrated the use of Raman spectroscopy for the characterization of the calcium sulphide, magnesium aluminate spinel, Al₂O₃, and calcium aluminate phases of CA, C3A, and C12A7 in a duplex oxide-sulphide system, and it offers potential for inclusion characterization in steel.