The infrared spectra (4 000 − 140 cm−1) of the L-alaninate complextrans-[Pt(L-ala)2] and its18( )-,15N, 2-d,3-d3- andN,N-d2-labelled analogues have been determined. Each spectral band has been assigned on the basis of the shifts induced by the various modes of isotopic labelling and by substitution of Pd(II) for Pt(II). The labelling study reveals that very few bands represent vibrationally-pure modes and that certain earlier assignments require revision.