In our model HIE reactions, isotopologue distribution as a function of time can be assayed by sampling reactions for analysis by mass spectrometry. A broad range of substrates were assayed in this way in the HIE of aryl substrates where both C-H bonds ortho to the directing group can be exchanged for C-D bonds. While in most substrates there is an initial buildup of d1-isotopologue population before significant quantities of the d2-isotopologue are detected, 2-phenylpyridine and 2phenylpyrimidine behave anomalously. Electronically modified pyridyl directing groups show different behaviour, implicating the binding of the directing group in the control of isotopologue distribution. Importantly, there is not a simple correlation between binding strength of the directing group and the isotopologue distribution during the reaction.