With the aim of shedding some light on the still scarcely investigated mechanism of transformation of imines in metal complexes, this study describes the investigation of the hydrogen–deuterium (H/D) exchange reaction of a bis[2-(pyridylmethylidene)-1-(2-pyridylmethylamine]iron(II) complex ([Fe(PMAP)₂]²⁺), following our previous work on a low-spin iron(II) complex bearing two molecules of S-2-pyridylmethylidene-1-(2-pyridyl)ethylamine. This complex has been proven to undergo successive transiminations in acetonitrile, yielding a bis[1-(2-pyridyl)ethylidene-2-pyridylmethylamine]iron(II) complex. In the analogous [Fe(PMAP)₂]²⁺ complex, a 1,3-hydrogen rearrangement occurs in a 10% deuterium oxide-acetonitrile-d₃ (D₂O–CD₃CN) solution. The H/D exchange reaction of [Fe(PMAP)₂]²⁺ was examined in the presence of various concentrations of 2,6-dimethylpyridine as a base in a 10% D₂O–CD₃CN solution at 45 °C, and the reaction mechanism was investigated.