The present invention relates to methods and reagents for [.sup.18F]-fluorination, particularly of peptides. The resultant .sup.18F-labelled compounds are useful as radiopharmaceuticals, specifically for use in Positron Emission Tomography (PET). Thus, a compound of formula .sup.18F-(Linker)-SH, such as a compound of formula (IV), (V), or (VI): .sup.18F--(CH.sub.2CH.sub.2O).sub.n--(CH.sub.2).sub.m--SH (IV) .sup.18F--(CH.sub.2).sub.p--SH (V) ##STR00001## may be reacted with an activated peptide as a means for .sup.18F-labelling.