Abstract: A process for preparing an alkyl aryl sulphonic acid comprising the steps of: (a) contacting an alkyl aromatic hydrocarbon with a gaseous sulphonating agent to produce (i) a first liquid reaction product comprising an alkyl aryl sulphonic acid and (ii) a gaseous effluent stream; (b) separating the first liquid reaction product from the gaseous effluent stream; (c) purifying the gaseous effluent stream to provide a cleaned gaseous stream and a second liquid reaction product; (d) recycling the second liquid reaction product to the first liquid reaction product produced after separation step (b) to produce a third liquid reaction product comprising alkyl aryl sulphonic acid; wherein the alkyl aromatic hydrocarbon is obtained by contacting an aromatic hydrocarbon with an olefin under alkylating conditions, and wherein said olefin is obtained by dehydrogenation of a Fischer-Tropsch derived paraffinic feedstock.