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Far-infrared transmittance measurements are performed on a series of isotopically labeled samples of the quasi-one-dimensional solids [Pt(en)z] [Pt(enzXz][C104]4, (en) =ethylenediamine and X =Cl, Br, or I.Pronounced shifts upon deuteration of the (en) ligands reveal that several absorption features previously attributed to localized vibrational modes of chain defects are in fact (en) ligand modes. Recognition of these ligand modes resolves the previous ambiguity over the assignment of the infrared-active chain phonons, permitting the unambiguous assignment of the 238.7-cm feature in the bromide material to the asymmetric stretch (v2) chain phonon, and strongly suggesting the 184.2-cm feature in the iodide is the v2 phonon of that material. A high-resolution examination of the chloride, prepared with both natural Cl isotopic abundance and with nearly pure "Cl, reveals a Cl isotopic fine structure that allows conclusive identification of the 359.1-cm feature as the v2 chain phonon. Lattice-dynamics calculations using a harmonic-linear-chain model with randomly distributed Cl isotopes yield good agreement with experiment and reveal that isotopic disorder leads to pronounced vibrational localization in PtC1, with the observed fine structure arising from modes residing on a few distinct sequences of isotopes occurring with high probability. The radical difference between the infrared fine structure of PtC1 and that previously reported for the Raman-active chain mode is found to result from differences in the dispersion curves for the two phonon branches, and allows indirect determination of these dispersion curves.PtX chain, as well as the carbonnitrogen backbones of the planar (en) ligands bound equatorially to each Pt. Not shown, for the sake of clarity, are the hydrogen atoms of the (en) ligands, two of which are bonded to each C and to each N atom, and the C104 counterions which lie between the chains and complete the crystal structure. If one considers only the onedimensional (X -Pt + -X . Pt . )"chain, lat-