The disclosed sulfur recovery system and process avoid some of the operational problems of conventional Claus plants and processes by replacing the reaction burner and furnace tube of a conventional Claus plant with a more efficient short contact time catalytic reactor assembly containing a SPOC.TM. catalyst that operates efficiently at elevated temperatures. Such Claus plant modification also reduces or avoids the negative effects of hydrocarbons, CO.sub.2 and ammonia in Claus sulfur plant feeds, and permits efficient processing of dilute to concentrated H.sub.2S feeds. The disclosed modification makes possible the expansion of Claus plant capacity at lower cost.