The mass spectra of a series of pentafluorophenyl derivatives of phosphorus have been examined. Bond-forming rearrangements were observed such as elimination of PF2 from the (C6F5)2P+ species giving (C6F4)2+, and elimination of successive molecules of FeF2 from [(C6F5)2PFe]2+. Some Group IV pentafluorophenyl derivatives and two pentachlorophenyl compounds were also examined.