The present invention provides a process for [18F]-fluorination of biomolecules containing a primary amino group such as proteins and peptides and in particular of peptides. The invention further provides reagents for this process, in particular 18F-labelled prosthetic groups for use in the preparation as well as non-labelled intermediates useful in the preparation of the [18F]-labelled prosthetic groups. [18F]-labelled compounds useful as radiopharmaceuticals, specifically for use in Positron Emission Tomography (PET) are also provided.