A feed gas stream containing hydrogen sulfide is subjected to a Claus reaction in a Claus furnace (16). Oxygen or oxygen-enriched air containing at least 80% by volume oxygen is supplied to the furnace (16) to support combustion of the hydrogen sulfide. Sulfur vapour is condensed out of the resultant effluent gas in a sulfur condenser (3). The sulfur depleted effluent gas is subjected to a plurality of stages of catalytic reaction of hydrogen sulfide in order to form further sulfur vapour in catalytic reactors (36, 44) and (52) with the sulfur vapour being condensed out of the gas in sulfur condensers (38, 46) and (54). The sulfur dioxide content of the resulting tail gas is reduced to hydrogen sulfide in reactor (6).