Sulfonation of oils

In a method for sulfonating aromatic petroleum oil feedstock with sulfur trioxide, the improvement, successful in avoiding the tough, intractable sludge which otherwise forms when sulfonating said feedstock with SO.sub.3 at atmospheric pressure, which consists essentially in (i) mixing at a temperature in the range of 110.degree.-200.degree. F. of an aromatic petroleum feedstock with a liquid sulfonic acid diluent, said diluent being present in the concentration of about 3% to about 50%, by weight, based upon the weight of the feedstock, (ii) vigorously stirring said mixture while gradually adding, over a period of about 10 minutes to about 3 hours, an amount of sulfur trioxide sufficient to sulfonate said aromatic petroleum oil feedstock at substantially atmospheric pressure, (iii) thereafter allowing the resulting product mixture of sulfonated feedstock oil and sludge to stand whereby said sludge settles out from the sulfonated oil, and (iv) separating said substantially sludge-free sulfonated oil from said sludge.