Glutaryl and adipoyl chlorides undergo facile condensation with the bis(silyl)phosphanes RP(SiMe3)2 (R = Me, nBu, tBu, Ph, Mes) to afford exclusively the phosphacycloalkyldiones (CH2)n(C[double bond, length as m-dash]O)2PR (n = 3, 4). Characterised spectroscopically and, for R = Ph, Mes (n = 3) crystallographically, the macrocycles are conformationally fluxional in solution and appreciably moisture sensitive. Though seemingly resistant to chemical oxidation at phosphorus, coordination is readily achieved, as illustrated by isolation of trans-[Pt(PEt3){P(Ph)(CO)2(CH2)3}Cl2] and a series of tungsten pentacarbonyl complexes, which are characterised crystallographically and by infrared and NMR spectroscopy. Together, these data suggest the macrocycles to be relatively weak σ-donors with no appreciable π-acceptor character.