Compounds of the formula (I) are disclosed:18F—(CHR)n(CH2)mCHO (I)in which n and m are independently 0 and 1 with at least one of n and m being 1, and R (if present) is a hydrogen atom or a methyl group, subject to the proviso that if n is 1 and R is methyl then m is 0. Synthesis of the compounds is described together with their use in radiolabelling reactions, e.g. for the radiolabelling of peptides to facilitate detection by Positron Emission Tomography (PET) imaging.The preferred compound is [18F]Fluoroacetaldehyde.
