A process for increasing the hydrogen content of a synthesis gas containing one or more sulphur compounds is described, comprising the steps of (i) heating the synthesis gas and (ii) passing at least part of the heated synthesis gas and steam through a reactor containing a sour shift catalyst, wherein the synthesis gas is heated by passing it through a plurality of tubes disposed within said catalyst in a direction co-current to the flow of said synthesis gas through the catalyst. The resulting synthesis gas may be passed to one or more additional reactors containing sour shift catalyst to maximize the yield of hydrogen production, or used for methanol production, for the Fischer-Tropsch synthesis of liquid hydrocarbons or for the production of synthetic natural gas.