A series of highly crowded symmetric and unsymmetric diphosphinomethanes R₂PCH₂PR′₂, important ligands in transition metal chemistry and catalysis, namely ^tBu₂PCH₂P^tBu₂ (dtbpm, 1), Cy₂PCH₂PCy₂ (dcpm, 2), ^tBu₂PCH₂PCy₂ (ctbpm, 3), ^tBu₂PCH₂PⁱPr₂ (iptbpm, 4) and ^tBu₂PCH₂PPh₂ (ptbpm, 5), has been prepared in high yields, using a general and convenient route, which is described in detail for 1. Other than 4, which is a colourless liquid, these compounds are crystalline solids at room temperature. Their molecular structures have been determined by single crystal X-ray diffraction, along with that of the higher homologue of 1, ^tBu₂CH₂CH₂^tBu₂ (dtbpe, 6). The solid-state structures of the dioxide of 1, ^tBu₂P(O)CH₂P(O)^tBu₂ (7), and of two phosphonium cations derived from 1, protonated [^tBu₂P(H)CH₂P^tBu₂]⁺ (8⁺) and the chlorophosphonium ion [^tBu₂P(Cl)CH₂P^tBu₂]⁺ (9⁺), are also described and show a distinct structural influence of the tetracoordinate P centres. The gas phase UV-photoelectron spectra of the diphosphines 1–6 have been measured. Their first two ionisation potentials are found to be nearly degenerate and all are in the low energy range from 7.5 to 7.8 eV. Comparison with related mono- and bidentate phosphines demonstrates that 1–6 are excellent σ-donors towards metals, in accord with their known coordination chemistry. Molecular geometries and electronic structures of the diphosphine systems have been studied by quantum chemical calculations and are compared to experiment. Unlike standard semiempirical methods (AM1, PM3, MNDO), which give rather poor minimum structures and seem inadequate for such sterically crowded systems, ab initio calculations (RHF/6-31G**) predict molecular geometries with reasonable accuracy and reflect the observed trends in experimental ionisation potentials.