Concentration dependence of the 31P NMR chemical shift of cyclenphosphine oxide, 1, indicates that a monomer-dimer equilibrium is operating in chloroform-d solution. This interpretation is supported by infrared spectroscopy. The high-field positions and the 31P NMR chemical shifts of the monomeric and dimeric forms (+10.4 ± 0.5 and -11.2 ± 0.5 ppm, respectively) suggest coordination of the pendent secondary amine group in these structures to the phosphorus center. Among the homologous cyclic phosphoramides, the smallest member, cyclenphosphine oxide, appears to be unique in this tendency for coordination to the phosphorus. Carbon NMR spectra of 1 show exchange of all of the carbon sites presumably through the mtermediacy of the hydroxyphosphorane 3.