The application relates to new substituted xanthines of general formula ##STR00001## wherein R.sup.1 and R.sup.2 are defined as in claims 1 to 11, the tautomers, the enantiomers, the diastereomers, the mixtures thereof and the salts thereof, which have valuable pharmacological properties, particularly an inhibiting effect on the activity of the enzyme dipeptidylpeptidase-IV (DPP-IV).