Like the phosphonitrilic chlorides (NPCl2)n, the phosphonitrilic fluorides (NPF2)n(n= 3–16) break down in the mass spectrometer into a mixture of cyclic and linear species. The parent ions, and more generally the parent series PnNnFx+, are always important products, and for (NPF2)5 this series forms over 70% of the total fragments. The larger fluorides tend to break down, possibly through transannular interaction, into cyclic fragments, when these are otherwise known to be stable; thus, fluorine atoms being neglected, P6N6→P3N3+ P3N3 and P7N7→ P3N3+ P4N4. The preferred mode of decomposition of the octameric fluoride is into P4N4 fragments, though P3N3+ P3N5 is also important. A comparatively high proportion of P3N3 fragments is obtained from (NPF2)9. In the larger rings, from (NPF2)10 upwards, the product specificity tends to be lost, because of the increased variety of possible stable cyclic fragments. More important, for n > 6, the variation in the yields of the smaller cyclic fragments is overshadowed by the increasing and exceptionally high yields, not only of the parent series, but of the parent ion within it, an effect attributed, by analogy with the behaviour of benzene and its homologues, to extensive electronic delocalisation. In addition to the main series of limiting compositions PnNnF2n+ and PnNn– 1F2n+ 2+, small amounts of ions of the type PnNn+ 2F2n+ 6+(n= 4–12) have been detected.