The infrared spectra (4000 − 140 cm−1) of the complexes [Cu(Hgg)X] (Hgg = monoanionic glycylglycinate anion, X = Cl, Br), [M(Hgg)X(H2O)] (M = Mn, Cu, Zn, X = Cl; M = Co, Ni, X = Cl, Br) and their15N-labelled analogues, are discussed in relation to their known or proposed structures. Firm assignments are presented for the majority of the internal ligand modes and for metal-ligand vibrations of the basis of the band shifts resulting from15N-labelling, metal ion substitution and halogen substitution.