The preparation of a series of methyl- and methoxy-fluoro-1,3,2,4-diazadiphosphetidines [(I) and (II) respectively] has been accomplished, in many cases by substitution reactions on heavily fluorinated diazadiphosphetidines using organometallic reagents.The ¹⁹F{¹H} and ³¹P{¹H} n.m.r. spectra of these compounds have been analysed, using subspectral techniques and iterative computer fitting. In only two cases axial â equatorial exchange may be slowed on the n.m.r. time scale at low temperatures; the reason for this lies in the concerted pseudorotation nature of the exchange. The chemical shift and coupling constant data show the influences of substituents and geometry. It has been demonstrated that ² J PP is positive, and its values can be reproduced by a product of parameters characteristic of the environment of each phosphorus nucleus.