A relatively large amount of SO.sub.2 effluent gas from a sulfonation process is converted to SO.sub.3 and recycled back to the sulfonation process as part of the SO.sub.3 sulfonating agent. The rest of the SO.sub.3 used as the sulfonating agent is derived from a sulfur burner located downstream of the sulfonation operation and in the recycle path of the SO.sub.2 in the effluent gas. Hydrocarbons in the effluent gas are converted to CO.sub.2 and H.sub.2 O in the sulfur burner. The H.sub.2 O and SO.sub.3 are removed from the gas stream at an SO.sub.3 absorber which forms rich oleum from which the SO.sub.3 is subsequently removed for use in the sulfonation operation, leaving lean oleum which is used at the SO.sub.3 absorber to remove H.sub.2 O and SO.sub.3 from the gas stream. Gases leaving the SO.sub.3 absorber include some SO.sub.3 and are passed through an H.sub.2 SO.sub.4 absorber along with water and lean oleum to produce concentrated H.sub.2 SO.sub.4.