In this domain, we have examined the competition between different directing groups in detail, initially using rutheniumcatalysed C-H arylation reactions as a model system, albeit with limited directing group scope. However, iridium-catalysed hydrogen isotope exchange (HIE) processes provide convenient and operationally simple model C-H activation systems for such studies, especially due to the wide scope and, generally, mild conditions of this reaction class. Related to this, we have recently used HIE systems to study the directing ability of various Lewis basic groups, noting differences in behaviour with Ir(I) catalysts bearing different ligand combinations (Scheme 1(a)). Furthermore, the data from directing group reactivity scales were used to qualitatively predict the DG 1 mol% [Ir] behaviour of reactions in which there are multiple possible directing groups (Scheme 1 (b)). Notably, Valero et al. have also qualitatively explored directing group strength in Ir(I)catalysed HIE, and used DFT calculations to rationalise the experimental data. As part of our earlier studies, we previously reported preliminary data from mass spectrometry analysis of samples withdrawn at different times during iridium-catalysed HIE reactions; 11 these analyses allowed the isotopologue composition (d0, d1, d2) to be quantified at each timepoint. These data suggested that for 2-phenylpyridine, there were typically two labelling events per binding event, since the concentration of phenylpyridine-d1 isotopologue remained low throughout the reaction. Given that the relationship between substrate binding and C-H activation is critical to the control of these transformations, we elected to study these aspects of the HIE process in significantly more depth.