The gas-phase molecular structure of a thiadiazole-containing expanded heteroazaporphyrinoid (C42H39N15S3) has been studied by a synchronous gas electron diffraction and mass spectrometric experiment and density functional theory calculations using the B3LYP hybrid method and cc-pVTZ basis sets. The molecule has an equilibrium structure of C3h symmetry with a planar macrocycle and the thiadiazole rings oriented in such a way that the sulfur atoms point outwards from the inner cavity. The unsubstituted macrocycle (C30H15N15S3) has been studied by DFT computations. An algorithm for building a complete set of internal coordinates, used in the computation of vibrational corrections, is also described.