Fouling in a refinery vessel, such as heat transfer equipment, used in a petroleum refinery operation and in which a refineable petroleum feedstock is at an elevated temperature and in fluid communication with the refinery vessel during a petroleum refinery operation, is reduced by providing in the refineable petroleum feedstock an additive comprising (i) a poly(butylenyl)benzene sulphonic acid; or, (ii) a poly(propylenyl)benzene sulphonic acid; or, (iii) a combination of a poly(butylenyl)benzene sulphonic acid and a poly(propylenyl)benzene sulphonic acid.