The molecular structures of a representative selection of medium-sized bicyclic diphosphines (i.e., diphosphanes) were studied by means of the ab initio second-order Moller-Plesset (MP2) method. The calculated results matched the available X-ray crystallographic data reasonably. A distinctive structural aspect is that all the 1,5-diphosphabicyclo[3.3.3]undecane systems, the 1,6-diphosphabicyclo[4.4.4]tetradecane free base, and its axially disubstituted derivatives adopt the out, out-conformation, whereas the monosubstituted [4.4.4] species prefer the out, in-arrangement. The intrabridgehead interaction was described in terms of polarity, bond order index, force constant, vibrational frequency, and nature of the critical point of the total electron density. The electronic structures of the [3.3.3] and [4.4.4] diphosphanes were theoretically investigated by means of their NMR spectroscopic properties. The chemical shifts and indirect nuclear spin-spin coupling constants, computed by density functional theory (DFT)-based methods, were in satisfactory agreement with the experimental values. In particular, the theoretical results yielded a consistent account of the strong orientational effects of the phosphorus lone pair and coordination on δ(P-31), J(P-31-P-31), and J(P-31-C) that show remarkable changes of sign and magnitude within this class of compounds.