All experiments presented here were done for a SABRE system using the IrCl(COD)(IMes) complex , where Imes=1,3-bis(2,4,6trimethylphenyl)imidazole-2-ylidene and COD= cyclooctadiene; activation of this pre-catalyst by hydrogenation of COD and addition of pyridine forms the main dihydride Iridium complex [Ir(H) 2 (IMes)(Py) 3 ] + , with Cl -as a counter ion (Py =pyridine). The structure of the SABRE complex is given in Figure . The SABRE process in this system is due to exchange of H 2 and Py between their free forms in solution and bound forms. It is important to note that in methanol solution there is also exchange between the main complex and two other complexes: one with a Cl -and one with an equatorial Py ligand replaced by a methanol solvent molecule. In these complexes the pH 2 -nascent protons are chemically non-equivalent, which strongly affects the spin conversion. To simplify the reacting system, we have replaced the Cl -counter-ion with PF 6 -, which does not bind to the complex. As previously described this can be achieved by adding AgPF 6 and removing the resulting AgCl precipitate from the sample. As a SABRE substrate, we used either N-labelled Py-d 5 or mixtures of N-Py-d 5 and 14 N-Py-d 5 , thus varying the abundance of the spin-1/2 N isotopes. Using a deuterated substrate allowed to simplify the spin system and, hence, to ease the optimization and interpretation of SABRE experiments.