Incorporation of a thiol reactive functional group at specific sites within an oligonucleotide allows for subsequent post-synthesis conjugation of the oligo with a number of different moieties such as fluorescent markers and biotin, depending on the application. Such labels need to be reactive towards the incorporated functional group: for example, thiols will react with iodoacetate and maleimide derivatives to form thioether linkages. The 3'-thiol-modifier CPGs with C3 and C6 S-S linkers can be used to introduce a 3'-thio functionality, with subsequent cleavage of the disulphide linkage affording the free thiol. This is then available for conjugation.