Unreacted sulfuric acid contained in a disulfonic acid product of an aromatic hydrocarbon that is produced from the reaction of the aromatic hydrocarbon or the mono-sulfonic acid of the aromatic hydrocarbon with a strong sulfonating agent is removed from the disulfonic acid product by reacting the sulfuric acid with additional aromatic hydrocarbon to produce the mono-sulfonic acid of the aromatic compound. The disulfonic acid product of the aromatic hydrocarbon containing sulfuric acid is reacted with the aromatic hydrocarbon at a molar ratio of aromatic hydrocarbon to sulfuric acid in the range of about 0.25 to around 2 and at a temperature in the range of 130.degree. to 200.degree. C. to obviate the need to remove any water formed during the course of the reaction.