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Ph2P(C5HMe4) (1) was found to undergo a prototropic rearrangement contrary to previous reports. The first detailed investigation of a cyclopentadienylphosphane to undergo such a reaction is presented. One of the two tautomers of 1 which were observed represents the kinetic product, the other one is the thermodynamic product. The two isomers exhibit pronounced differences with respect to their chemical reactivity. Thermodynamic and kinetic data of the isomerisation process were determined. A novel, electron rich cyclopentadienylphosphane, the phosphorus diamide (Me2N)2P(C5HMe4) (2), was synthesised and was also found to undergo a prototropic rearrangement. The characteristics of the isomerisation process are fully consistent with the findings on 1. The molecular structure of the kinetic product of 2 was established by means of single crystal X-ray diffraction analysis. Me2P(C5H4tBu) (3) is presented as another novel cyclopentadienylphosphane. The mixture of isomers of 3 in thermodynamic equilibrium was investigated in detail with respect to the molecular structures of the tautomers. The results of this study suggested a reinvestigation of the parent compound Ph2P(C5H4tBu) (4) and findings differing from an earlier report in some details are presented.