In conclusion, we have identified a previously non-detectable reversible exchange reaction of chlorine and methanol with Ir-based organometallic catalyst systems used for SABRE hyperpolarization. Furthermore, we show that the chemical-exchange saturation transfer (CEST) scheme is an effective and useful tool to experimentally probe ligand exchange reactions in organometallic complexes. CEST NMR opens the way to gaining experimental insight on reaction dynamics in catalysis and biochemistry (e.g., for studying enzymatic reactions, such as hydrogenase ) even in cases where such reactions are inaccessible by conventional or hyperpolarized NMR. We also demonstrate the role of exchanging, lowconcentrated intermediate complexes on the spin dynamics of the free pH2 pool. By removing the complex [Ir(Cl)(IMes)(H)2(Py)2] from solution, we show an increase of the singlet state lifetime by more than an order of magnitude, demonstrating a promising new strategy of optimizing SABRE systems for in situ hyperpolarization at high magnetic fields.