A novel, storage-stable, free-flowing, solid product consisting essentially of an alkyl benzene sulphonic acid having 1 to 4 aliphatic carbon atoms and from 7 to 11 % water based on the weight of the product, said alkyl benzene sulphonic acid comprising at least 90 % by weight of a single isomer based on the total weight of organic material in the product, is prepared by drying a storage-unstable, solid alkyl benzene sulphonic acid having more than 11 % by weight water, said drying being effected at sub atmospheric pressure and at a temperature below the softening point of the product.