Micro-scale polarization data from small surface areas of stainless steels with either CeS or MnS inclusions was measured in NaCl and Na2SO4 solutions to elucidate the dissolution and pit initiation behavior of both types of inclusions. Stable pitting was initiated at the CeS inclusion at potentials which exceeded the dissolution potential region of the CeS inclusion, whereas a stable pit at the MnS inclusion was initiated in the dissolution potential range of the MnS inclusion. Thermodynamic calculations indicated that the Ce3+ ions are likely to be produced by the dissolution of the CeS inclusion. In the micro-scale polarization of a small area with a MnS inclusion, stable pitting at the MnS inclusion was inhibited in NaCl solutions containing Ce3+. The formation of a shallow trench was observed at the MnS inclusion/steel matrix boundary in the Ce3+-containing solution, whereas a deep trench was formed in the Ce3+-free solution. It is suggested that the Ce3+ ions inhibit trench formation at the MnS/steel matrix boundary, resulting in improved pitting corrosion resistance at sulfide inclusions.