Immobilizing bifunctional phosphines with ethoxysilane groups on silica often leads predominantly, and sometimes quantitatively, to P(V) side products that occupy space on the surface but cannot bind metal complexes. Although this side reaction is well-known, and dreaded because it leads to leaching of adsorbed catalysts that are not bound covalently, the exact nature of the surface-bound side product was not yet known. With the help of polycrystalline model compounds of the types [R3PEt]X-+(-) and [R3POEt]X-+(-) (R = alkyl, aryl; X = Cl-, Br-, I-, BF4-) and their solid-state NMR characteristics [delta(P-31), CSA], it is demonstrated that the side product is an ethyl phosphonium salt bound to the surface by a siloxide anion, [R3PEt](+)[Si-O](-).