A first example of experimental study of molecular structure of free porphyrino metal complexes is given. Gas-phase electron diffraction was applied for the molecular structure determination of octamethylporphyrin copper(II), C 28 H 28 N 4 Cu, at an effusion cell temperature of 674(10) K. The molecule was found to possess a geometric configuration of D 4h symmetry with the following main internuclear distances and valence angles: r h1 (Cu–N)=2.013(5)Å, r h1 (C(N)–N)=1.378(4)Å, r h1 (C(N)–C)=1.454(3)Å, r h1 (C–C(Me))=1.503(3)Å, r h1 (C(Me)–C(Me))=1.372(3)Å, r h1 (C(N)–C(α))=1.390(3)Å, ∠Cu–N–C(N)=127.1(1)°, ∠C(N)–N–C(N)=105.7(2)°, ∠N–C(N)–C(Me)=110.8(2), ∠C(N)–C(Me)–C(Me)=106.4(2)°, ∠C(N)–C α –C(N)=125.5(3)°, ∠C(H)–C(Me)–C(Me)=128.1(4)°. Quantum chemical calculations, DFT/B3LYP with 6-31G* and TZV (d,p) basis sets, reproduce the experimental bond distances with accuracy within 0.02Å. NBO-analysis of electron density distribution was carried out, the energy of donor–acceptor interaction was estimated, and it was shown that the back donation makes a minor contribution to this interaction.