The infrared spectra of cis-(Me3P)2PtBr2, trans-(Me3P)2Ptl2, and Et3P are discussed with a view to assisting in the identification of platinum–phosphorus stretching frequencies in triethylphosphine complexes. For trans-(Et3P)2PtCl2, the infrared-active metal–phosphorus stretching frequency is found at 415 cm.–1 while the symmetrical stretching frequency was observed in the Raman spectrum with a shift of 443 cm.–1. For cis-(Et3P)2PtCl2, the two infrared-active stretching frequencies are found at 442 and 427 cm.–1. This distinction has been used to determine the stereochemistry of a number of derivatives (Et3P)2PtX2(X = Br, I, NCS, CN, Ph, or C6F5). It is found that the infrared-active metal–phosphorus stretching frequencies in nickel and palladium compounds are within 6 cm.–1 of the stretching frequencies for the corresponding platinum compounds. By this means, a cis-bistriethylphosphine derivative of palladium, cis-(Et3P)2Pd(C6F5)2, has been identified for the first time. All the other palladium and nickel derivatives were shown to have the trans-configuration. Some unsymmetrical derivatives, (Et3)2MXY (M = Ni, Pd, or Pt; X = Cl, Br, I, or Me; Y = Ph or C6F5), have also been investigated. In spite of the fact that the trans-derivatives are not centrosymmetric, only one metal–phosphorus absorption could be identified.