Derivatives of phenoxaphosphine (I) do not behave under electron impact like diphenyl ethers: they do not lose CO, while loss of CHO was observed. The first step is the breakage of the P-R bond, followed by elimination of the phosphorus atom or—after rearrangement — a PO group. Methyl groups are either eliminated or cause the formation of tropylium structures. In halogen derivatives of I, not only the elimination of CHO, but also of COHal has been observed. In phenyl-di-(o-phenoxyphenyl)-phosphine (XXVI), a C-O bond is broken in preference to a C-P bond, and the 10,10-diphenylphenoxaphosphonium ion (XXVIII) is formed.