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The infrared spectrum of PtCO isolated in solid argon has been reinvestigated. Isotopic data on ν1, ν2, ν3, 2ν1, and ν1 + ν3 have been measured in the near- and far-infrared regions. This enables a complete harmonic force-field calculation based on a linear geometry, in agreement with all theoretical predictions. Comparisons of spectroscopic parameters for the NiCO, PdCO, and PtCO series experimental and theoretical binding energies are also presented. These results show that the perturbations of the CO ligand (CO bond force constants and ν1 frequency shifts) are unreliable indicators of the evolution of the metal−ligand interaction energies, contrary to the metal−carbon force constant. Comparison with theoretical predictions of the fundamental frequencies indicates a systematic underestimation of the bending frequencies, especially sensitive for the heavy atom monocarbonyls. Similarly, the IR spectrum of Pt(CO)2 has been studied, enabling the determination of the ν1, ν2, ν3, ν4, and ν6 fundamentals of this molecule and a discussion of the evolution of the Pt−C and C⋮O force constants with the coordination number. Complementary results concerning Pt(CO) n (n = 3, 4) are also presented.