Vegetable and mineral oils and the like are purified by kieselguhr or colloidal silica acid, impregnated with very small quantities of sulphuric acid. In an example kieselguhr and sulphuric acid of 10 per cent strength are mixed to a pulp, and the pulp is pressed to remove excess of acid and is then dried by heating with agitation until sulphurous fumes are evolved. 20 kilograms of the product are used to purify 1000 kilograms of Russian spindle oil. The Specification as open to inspection under Sect. 91 (3) (a) comprises also the use of similarly impregnated carbon or other porous substance. This subject-matter does not appear in the Specification as accepted.