Petroleum sulfonates are obtained by continuously contacting whole crude oil or topped crude oil with gaseous sulfur trioxide (contains as a diluent sulfur dioxide and light hydrocarbon vapor) in a reaction zone operated at a temperature of 120.degree.-250.degree. F. and a pressure of 3-50 psia. The reaction product is then passed to a vapor-liquid separating stage where a vapor stream is separated and a portion of it is recycled back to the reaction zone; a liquid stream is separated from the separating stage and a portion of it is recycled back to the reaction zone. The remaining portion of the liquid stream is neutralized with a monovalent inorganic base to obtain the petroleum sulfonate. The petroleum sulfonates are particularly useful to recover crude oil from subterranean reservoirs.