A method of making a set of labelled compounds by the use of a preferably particulate support, comprises dividing the support into lots, performing a different chemical reaction on each lot of the support, e.g. to couple a chemical moiety to that lot of the support, tagging a fraction of each lot of the support with a different label, and combining the said lots of the support. The steps are repeated several times, preferably to build up oligomer molecules carrying labels which identify the nature and position of a monomer unit of the oligomer, and which are releasable from the support. Preferred labels, which are releasable from the compounds by cleavage to provide charged groups for analysis by mass spectrometry, are groups of the trityl (trimethylphenyl) family. Also claimed are libraries of these labels and their use in assays and nucleic acid analysis methods.