Abstract: An improved and modified Claus process control system utilizing first and second coulometric titrators for automatically controlling the amount of oxygen used in burning a portion of the hydrogen sulfide (H.sub.2 S) in flue gases. The first and second titrators are fed sulfur dioxide (SO.sub.2) and H.sub.2 S scrubbed samples, respectively, of the flue gases downstream of the oxygen injection point. The first and second titrators produce H.sub.2 S and SO.sub.2 analog output signals, respectively, which are used to control the rate of flow of oxygen or air injected in a manner such that the samples of the flue gases contain H.sub.2 S in a concentration which is approximately or exactly equal to twice the concentration of the SO.sub.2 therein.