PELDOR and F ENDOR measurements require the presence of two or one paramagnetic spin labels, respectively. Several methods for site-directed spin labelling of nucleic acids at the base, ribose, or phosphate exist. Custom synthesised RNA sequences with phosphorothioate modification are commercially available as is the spin label precursor for labelling at the phosphate backbone (Scheme 1) making this labelling approach readily accessible. This procedure leads to the introduction of a diastereomeric pair. The spin labelled RNA is therefore expected to display both diastereomers of the phosphothiotriester.