A process for treating an exhaust gas contaminated with dust, Hg, NO.sub.x, and SO.sub.x is provided by adding a mercury chlorinating agent, such as, for example, HCl, NH.sub.4 Cl, or Cl.sub.2, and ammonia to the exhaust gas so that the mercury in the exhaust gas is converted to HgCl.sub.2. The dust, HgCl.sub.2, NO.sub.x, NH.sub.3, and SO.sub.x containing the exhaust gas may then be passed through a reactor containing a catalyst such as, for example, V.sub.2 O.sub.5 and/or MoO.sub.3 supported on TiO.sub.2, which promotes the reaction between the NO.sub.x and NH.sub.3 to result in the formation of N.sub.2 and H.sub.2 O. The HgCl.sub.2 and SO.sub.x containing exhaust gas is then passed through a scrubbing tower where the gas is scrubbed with aqueous solutions of absorbents such as CaCO.sub.3 and Ca(OH).sub.2, so that HgCl.sub.2 and SO.sub.x are removed from the exhaust gas. The cleaned exhaust gas is discharged through a stack.