The invention relates to 3- or 4-monosubstituted phenol and thiophenol derivatives of formula (I) and to processes for the preparation of, intermediates used in the preparation of, compositions containing and the uses of, such derivatives. Said 3- or 4-monosubstituted phenol and thiophenol derivatives are H.sub.3 ligands and are useful in numerous diseases, disorders and conditions, in particular inflammatory, allergic and respiratory diseases, disorders and conditions. ##STR00001##