Despite the growing interest in directed Hydrogen Isotope Exchange (HIE) reactions, the existing synthetic protocols mainly rely on noble metals and the choice of the deuterium source remains unclear. Herein, we developed different reaction conditions to enable the site-selective Cp*Co-catalyzed deuteration of aromatic compounds containing up to twenty prevalent coordinating directing groups, with two readily available deuterium sources, D2O and CD3OD. Our protocol enables the labelling of diverse complex molecules. By modelling the data obtained in the reaction scope through multivariable linear regression (MLR), we suggest that a shift in the substrate-dependent mechanism dictates the need for a specific source of deuterium.