The invention provides a process and system for purification of an exhaust gas stream from a combustion engine containing hydrocarbons, soot, carbon monoxide and sulphur dioxide. The process comprises the steps of oxidising the hydrocarbons and part of the soot in a first reactor in the presence of a first catalyst active in oxidising hydrocarbons and soot without oxidising sulphur dioxide and forming a partly purified exhaust gas stream, of cooling of the partly purified exhaust gas stream, of converting the carbon monoxide of the partly purified exhaust gas stream in a second reactor in presence of a second catalyst active in oxidising carbon monoxide without oxidising sulphur dioxide and of withdrawing a purified exhaust gas stream.