During the combustion of sulfur with air for producing sulfur dioxide, sulfur is burnt in a first portion of the combustion furnace under substoichiometric conditions. The sulfur dioxide formed in the first portion and unburnt sulfur are supplied to a second portion of the furnace adjoining the first portion, in that they are subjected to post-combustion with air. This provides for the productionof sulfur-dioxide- containing gas free from nitrogen oxide.