The cyclic phosphonitrilic chlorides (NPCl2)n(n= 3–8) break down in the mass spectrometer into a mixture of cyclic and linear products, the types of compound formed being those familiar in the general chemistry of these compounds. Species with an even number of electrons are favoured over radicals. The linear species decrease in stability with increase in chain-length. The larger cyclic compounds (n= 6–8) tend to decompose preferentially into those cyclic fragments which are otherwise known to be stable. The ions P5N6Cl6+, P6N7Cl6+, P6N7Cl8+, P7N8Cl8+, P7N8Cl10+ fall outside the general pattern, in that they contain more nitrogen than phosphorus atoms, and are likely to have condensed ring structures.