The bifunctional phosphines Ph(2)P(CH2)(3)Si(OEt)(3) and Ph(2)P(CH2)(2)Si(OEt)(3) are commonly used in order to bind metal complexes to support materials. When these phosphines are grafted on the silica surface, a side reaction takes place and a P(V) species is generated in considerable amount, which is no longer able to bind transition metal catalysts. This P(V) species is not the immobilized phosphine oxide, in contrast to a widespread belief. Instead, it has a cyclic, surface-attached, oxyphosphorane type structure with a pentacoordinated P(V) atom. This assumption is corroborated mainly by (a) P-31 NMR spectroscopic comparison with surface-bound phosphine oxides and phosphonium salts, as well as dipolar dephasing P-31 NMR experiments, and (b) independent synthesis and P-31 and C-13 NMR spectroscopic characterization of a surface-attached model compound. A pseudo-Wittig type reaction mechanism for the formation of the P(V) species is proposed, on the basis of the observation that both, Oft groups and silica surface, are indispensable prerequisites for this side reaction to take place. The reaction can either be used to attach any monofunctional phosphine to the silica surface as a P(V) species or avoided by mild reaction conditions or application of bifunctional phosphines without Si(OEt)(3) groups like Ph(2)P(CH2)(4)OH.