In a process for producing sulphuric acid from roasting gas containing SO.sub.2, H.sub.2 O and Hg, comprising the steps of(a) drying the gas with H.sub.2 SO.sub.4(b) subjecting the gas to a catalytic oxidation treatment so as to produce a SO.sub.3 rich gas, and(c) absorbing the SO.sub.3 in concentrated sulphuric acid, while maintaining the concentration of this acid at its initial value by addition of diluted sulphuric acid, the gas from step (b) is scrubbed with oleum, thereby producing in step(c) H.sub.2 SO.sub.4 with less than 0.1 mg/l of Hg.