This work is devoted to structural and vibrational studies of N-alkyloxamates and their complexes. The structures of MeHNCOCOOK and [Cu(EtHNCOCOO)2] have been determined by single-crystal X-ray crystallography. The potassium salt crystallizes in the monoclinic space group P21/n and the copper(II) complex in the triclinic space group P1̄. The lattice constants are a=3.882(2), b=10.175(1), c=13.004(1), β=97.04(1)° for MeHNCOCOOK and a=5.013(1), b=6.996(1), c=8.575(1)Å,α=73.54(1), β=85.66(1), γ=71.22(1)° for [Cu(EtHNCOCOO)2]. The salt has a trans secondary amide group and the K− ion is surrounded by seven O atoms belonging to six different N-methyloxamate(-1) moieties. The ligand EtHNCOCOO− behaves as a bidentate chelate in the trans square planar copper(II) complex with ligated atoms being the amide and one of the carboxylate oxygens. The copper(II) complex was characterized by its room-temperature magnetic susceptibility and solid-state d–d spectrum. A detailed comparison of the vibrational spectra (FT-IR, FT-Raman) of [Cu(EtHNCOCOO)2] with the corresponding potassium salt, EtHNCOCOOK, is presented. The vibrational data are discussed in terms of the nature of bonding and the known structure of the copper(II) complex. Quantum-chemical calculations were performed to obtain the equilibrium geometries and torsional energies for an isolated oxamate(-1) ion (H2NCOCOO−) and its hypothetical square planar zinc(II) complex.