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A series of Mn-Ce/SAPO-34 catalysts were prepared to study the catalytic oxidisation of elemental mercury (Hg0). Sulphur tolerance and SO3 formation over the catalyst were studied further. Hg0 was transported by compressed air from PSA Cavkit. NO, SO2, and NH3 are standard gases, and H2O is produced by gas carrying. Mn could incorporate into the cerium oxide lattice to form capping oxygen and well-dispersed high valance manganese ions after the addition of Ce, which was conducive to NO removal and Hg0 oxidisation. 9 Mn-9Ce showed the best performance regarding Hg0 conversion, achieving more than 92% Hg0 conversion efficiency at 50–300 °C. The sulphur resistance of the Mn-based catalyst was significantly improved after the addition of cerium due to the high affinity of Ce for SO2, and the relative content of HgSO4 was exceeded 72% on the 9 Mn-9Ce catalyst with SO2; SO3 formation over the 9 Mn-9Ce decreased by 17% compared with the 9 Mn. H2O not only reduced the available active site, but also decreased the oxidation rate of SO2. The active sites were preferentially occupied by NH3 rather than Hg0 and SO2, generated NH4+ occupied cation vacancies. Therefore, both H2O and NH3 have inhibitory effects on Hg0 conversion and SO3 formation.