The degradation of paper artefacts by iron-gall inks is caused by two ingredients in the ink: sulphuric acid and iron (II) sulphate. Whereas sulphuric acid catalyses the hydrolysis of cellulose, iron (II) ions catalyse its oxidation. Both degrading agents, sulphuric acid and iron (II) sulphate are water soluble. A changing RH during storage is expected to cause migration of the agents, thereby spreading ink corrosion throughout the paper, X-ray fluorescence analysis of the iron and sulphur distributions across the ink lines of artificially aged test samples showed that sulphuric acid moved out of the inked areas, whereas iron did not. No iron migration could be observed with two ink samples of a 16th-century manuscript. With one ink, which had penetrated the paper, sulphur had migrated. With the other ink, which had not penetrated the paper, sulphur had not migrated. It could already have been present in the ink as calcium sulphate before application.