Herein we qualitatively examine the relationship between reaction rate and reaction selectivity in iridium-catalysed hydrogen isotope exchange (HIE) reactions directed by Lewis basic functional groups. We have recently developed a directing group scale that allows semi-quantitative predictions of Lewis base directed selectivity in HIE, formally ranking ‘relative rates’ determined from a structured set of competition experiments. Here, we show that selectivity and rate are in fact not correlated, but that different types of behaviour emerge in competition experiments and that the observed behaviour can be predicted from our established selectivity scale.