The central phosphorus atom of a novel hydroxyl-functionalized triarylphosphane was shown to reversibly insert into one of the molecule's O-H bonds, which forms the basis for a tautomeric lambda(3)/lambda(5)-phosphane equilibrium. For the first time, this equilibrium was detected for a lambda(3)-triarylphosphane and the underlying dynamic process was elucidated by NMR spectroscopy. On the basis of reactivity studies, a nucleophilic character was assigned to the minor species present in solution, the lambda(3)-phosphane. Upon methylation, for example, the lambda(3)-form was selectively removed from the equilibrium and converted to the corresponding phosphonium salt. However, upon generation of an alkoxide via proton abstraction, the electrophilic character of the lambda(5)-phosphane in the equilibrium became evident since the alkoxide was found to attack the molecule's phosphorus atom. This intramolecular reaction led to the selective formation of a new anionic lambda(6)-hydridospirophosphane.