A homologous series of bis-diphenylphosphine oxides (C 6 H 5 ) 2 PO-(CH 2 ) n PO(C 6 H 5 ) 2 (with n 2 ± 8; denoted 2 ± 8] have been investigated to explore the effects of a range of competing and cooperative intermolecular and intramolecular interactions on the structural properties in the solid state. The important factors influencing the structural properties include intramolecular aspects such as the conformation of the aliphatic chain and the intramolecular interaction between the two P O dipoles in the molecule, and intermolecular aspects such as long-range electrostatic interactions (dominated by the arrangement of the PO dipoles), CÀH ´´´O interactions, CÀH ´´´p interactions and p ´´´p interactions. Compounds 3 and 5 could be crystallized only as solvate co-crystals (3 ´water and 5 ´(toluene) 2 ], whereas the crystal structures of all the other compounds contain only the bis-diphenylphosphine oxide molecule. The crystal structures have been determined from single-crystal X-ray diffraction data, with the exception of 7 (which has been determined here from powder X-ray diffraction data) and 4 (which was known previously). The compounds with even n represent a systematic structural series, exhibiting characteristic, essentially linear P O ´´´P O ´´´P O dipolar arrays, together with C À H ´´´O and C À H ´´´p interactions. For the compounds with odd n, on the other hand, uniform structural behaviour is not observed across the series, although certain aspects of these crystal structures contribute in a general sense to our understanding of the structural properties of bis-diphenylphosphine oxides. Importantly, for the compounds with odd n, there is ªfrustrationº with regard to the molecular conformation, as the preferred all-anti conformation of the aliphatic chain gives rise to an unfavourable parallel alignment of the two PO dipoles within the molecule. Clearly the importance of avoiding a parallel alignment of the PO dipoles becomes greater as n decreases. Local structural aspects (investigated by high-resolution solid-state 31 P NMR spectroscopy) and thermal properties of the bis-diphenylphosphine oxide materials are also reported.