Abstract: Polymers are randomly labelled with labelling groups such as fluorophores, by a process of creating free radicals on the polymer in the presence of a stable free radical, such as an aminooxyl compound, so that the stable free radical group bonds to the polymer in random fashion. Labelling groups such as fluorophores are attached to the stable free radical groups, before or after they are attached to the polymer. The process allows labelling of polymers having no reactive functional groups, it can also be applied to the labelling of nucleic acids, for use in conjunction with a PCR chain extension sequencing process, to allow the sequencing of target nucleic acids of high molecular weight.